How to Set Up GST/QST in Your POS System or E-Commerce Platform — Montreal CPA Firm

Setting up GST and QST correctly in your POS system or e-commerce platform is critical for accurate tax collection in Quebec. Whether you use Shopify, Square, Lightspeed, Clover, WooCommerce, Stripe, or any other payment system, incorrect tax settings led to under-collected QST, over-collected GST, inaccurate invoices, refund disputes, and — most dangerously — Revenue Québec reassessments. Quebec has one of the most complex sales tax structures in North America because QST is calculated on the GST-inclusive amount, and most platforms built outside Canada do not apply this rule by default. This guide explains how to configure your POS or e-commerce platform properly using the same professional tax-compliance framework Mackisen CPA Montreal uses for clients.

Why This Matters
If your POS or e-commerce system calculates GST/QST incorrectly, every transaction you process becomes a tax error.
Under-collected QST → You must pay the difference out of pocket
Over-collected tax → Customers may legally demand refunds
Incorrect invoices → GST/QST filing discrepancies
Incorrect tax setup → Audit triggers and refund freezes
Tax settings are not optional — they determine your compliance risk and financial exposure.

Legal and Regulatory Framework
GST calculations follow the Excise Tax Act,

QST calculations follow Quebec’s Tax Administration Act,
and all systems must respect the unique Quebec rule:
QST = 9.975% applied on the GST-inclusive amount.
Hickman Motors (1997, SCC) places responsibility on the taxpayer, not the software.
Canderel (1998, SCC) confirms filings must reflect commercial reality.
Lac d’Amiante (2001, SCC) confirms RQ may reassess for incorrect or inconsistent tax collection — even years later.

A Quebec business sets up Shopify with default U.S. sales tax settings. Shopify calculates QST on the subtotal, not the GST-inclusive price. After two years, Revenue Québec audits the online sales. They discover the business under-collected QST by $22,000. Shopify is not responsible — the business is. Refunds freeze. A reassessment arrives with penalties and interest. The business must now pay thousands of dollars out of pocket because the platform was not configured for Quebec tax law. This scenario is extremely common.

Learning Section: Understanding How POS/E-Commerce Systems Treat Taxes
Most systems are built for the U.S. market
They assume GST = PST logic (not true for QST)
They often treat tax as one combined value
They do not default to GST → then QST on top
They do not ask where customers are actually located
They miscalculate HST for other provinces
They cannot automatically detect IP/location for digital products
You must manually configure the settings to comply with Quebec tax laws.

SEO Learning: Primary and Secondary Keywords
Primary: set up QST Shopify, configure GST POS Quebec, Quebec e-commerce tax setup.
Secondary: GST QST settings Square, Shopify QST calculation, POS tax configuration Montreal.

Step-by-Step: How to Set Up GST/QST in Your POS or E-Commerce System

Step 1: Turn on GST (5%)
GST must be applied to all taxable sales to Quebec customers and many out-of-province customers.
In your POS/e-commerce system, create a GST tax code:
GST = 5%

Step 2: Turn on QST (9.975% on GST-inclusive amount)
In Quebec, QST is applied after GST has been added, meaning QST is not based on the subtotal but on the new total:
Subtotal → add GST → apply QST
Your tax system must follow:
QST = 9.975% × (Subtotal + GST)
If the platform cannot apply QST correctly, it must be manually configured through combined tax rules or custom calculation.

Step 3: Activate Quebec as a Tax Region
Ensure your platform recognizes Quebec-specific tax laws.
Many systems automatically apply GST but NOT QST unless activated.

Step 4: Enable Multi-Province Tax Rules
Ontario customers → HST
Western Canada customers → GST only
Quebec customers → GST + QST
U.S. customers → usually 0%
International customers → usually 0%
The platform must apply these rules based on shipping, billing, or IP address.

Step 5: Test Your Invoices
Manually test:
$100 sale
GST should be $5
QST should be $10.47
Total = $115.47
If the system gives any other result — it’s wrong.

Step 6: Reconcile Monthly
Compare POS/e-commerce tax reports with:
Bank statements
Merchant processor deposits
Accounting system GST/QST ledgers
This ensures filing accuracy.

Documentation Section
System tax settings screenshots
Invoices generated from POS/e-commerce
Shopify/Stripe/Square tax reports
Merchant processor statements
Reconciliation worksheets
Sales documentation by province
Customer location evidence (IP or shipping)
All must be kept for six years.

Common Mistakes Made by Businesses
Using default U.S. tax settings
Charging only GST and forgetting QST
Applying QST on subtotal instead of GST-inclusive total
Failing to configure multi-province tax rules
Incorrect HST setup for Ontario/Atlantic provinces
Mixing digital tax rules with physical goods
Not reconciling merchant processor data
Not reviewing invoices monthly
These errors trigger QST reassessments and refund denials.

Winning With Revenue Québec and Canada
Mackisen configures your POS or e-commerce system correctly, ensures calculations follow Quebec law, and tests the system monthly for errors. We reconcile all tax accounts, identify under-collection or over-collection, correct filings, and prepare audit-ready documentation. Our team prevents the tax errors that automated platforms frequently create.

Mackisen Service Hub
We provide:
Tax configuration for Shopify, Square, Stripe, Lightspeed, Clover, and WooCommerce
GST/QST compliance setup
Monthly reconciliation
Digital and interprovincial tax mapping
Correction and amended filings
Audit-ready reporting
Refund recovery for incorrectly collected tax
Complete POS/e-commerce system optimization

Why Mackisen
With more than 35 years of CPA, digital commerce, and audit experience, Mackisen CPA Montreal ensures your GST/QST is calculated correctly across every platform you use. We protect your business from reassessments, under-collection liabilities, platform errors, and long-term tax exposure — ensuring your sales remain compliant, profitable, and audit-safe.