How to Avoid Common GST/QST Filing Errors — Montreal CPA Firm Near You

GST and QST filing errors are among the most common — and most expensive — mistakes Quebec businesses make. Even a small error in your GST/QST return can lead to reassessments, refund delays, penalties, and interest. Revenue Québec and the CRA both use automated systems to detect inconsistencies between your sales reports, bank deposits, merchant processor payouts, and prior filings. When they find discrepancies, your return is flagged for review or audit. This guide explains the most common GST/QST errors and how to avoid them using the same precision-based system that Mackisen CPA Montreal applies to protect businesses from costly tax issues.

Why Filing Errors Are Dangerous
A single mistake can trigger:
Refund freezes
Additional document requests
Reassessments
Penalties and daily interest
Compliance flags for future periods
Full audits
Errors often compound over multiple periods, creating thousands of dollars in unexpected liabilities. Correcting mistakes proactively is far safer than waiting for RQ or CRA to detect them.

Legal and Regulatory Framework
GST filing errors fall under the Excise Tax Act; QST errors fall under Quebec’s Tax Administration Act. Hickman Motors (1997) confirms the taxpayer bears the burden of proving accuracy. Canderel (1998) requires filings to reflect economic reality. Lac d’Amiante (2001) gives Revenue Québec the authority to reassess GST/QST filings for missing documentation or incorrect numbers.

What Happens When Filing Errors Go Unnoticed
Imagine you file your GST/QST return manually and accidentally misreport your sales by $20,000 due to an incorrect Excel formula. Your refund is processed — or so you think. Weeks later, CRA requests additional documentation because their system detects mismatches between your GST collected and merchant processor deposits. While reviewing the documents, they uncover more errors in previous periods. Now multiple returns are under review, your refunds are frozen, and your business is under stress. This is how a single overlooked mistake spirals into a multi-year review.

Learning Section: The Most Common GST/QST Filing Errors
Incorrectly reporting total sales
Misclassifying exempt, zero-rated, and taxable sales
Failing to charge QST on GST-inclusive amounts
Claiming ITCs/ITRs without proper invoices
Missing proof of payment
Incorrect QST calculation on digital or interprovincial sales
Double-counting expenses
Forgetting to self-assess QST on out-of-province purchases
Incorrect GST/HST on Ontario or Atlantic sales
Misreporting Shopify, Amazon, PayPal, or Stripe totals
Failing to reconcile merchant processor deposits
Incorrect filing periods
All of these errors are detectable by CRA/RQ’s internal cross-matching tools.

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How to Avoid GST/QST Filing Errors
Step 1: Reconcile revenue monthly
Match sales in your books to merchant deposits and bank statements.
Step 2: Verify tax codes
Ensure every item is coded as taxable, zero-rated, or exempt correctly.
Step 3: Validate ITCs and ITRs
Check invoices for GST/QST numbers and proof of payment.
Step 4: Review QST formula
Ensure QST is applied on GST-inclusive amounts.
Step 5: Reconcile tax payable accounts
Match GST/QST collected vs remitted.
Step 6: Audit platform settings
Verify Shopify, Square, Stripe, PayPal settings monthly.
Step 7: File early
Avoid last-minute errors.
Step 8: Save filing confirmation receipts
Proof of filing protects your business during disputes.

Documentation Section: What You Must Keep Preventing Errors
Sales invoices
Purchase invoices
Proof of payment
Merchant processor reports (Shopify, Stripe, PayPal, Square)
Bank statements
GST/QST reconciliation worksheets
Digital export logs (for zero-rated items)
Tax configuration screenshots
These documents must be kept for at least six years.

Common Mistakes Businesses Don’t Realize They Are Making
Using U.S.-based tax settings on Shopify
Accidentally applying U.S. “VAT-style” tax logic
Forgetting to charge QST on services delivered in Quebec
Charging QST to out-of-province customers
Failing to claim QST on rent or utilities
Incorrect proration of mixed-use expenses
Treating digital sales incorrectly
Not updating software after tax law changes
Assuming zero sales = no filing required
Filing the wrong statutory period
These are the exact issues RQ and CRA look for in audits.

Winning With Revenue Québec and Canada
Mackisen eliminates filing errors before returns are submitted. Our review includes:
Reconciliation of sales, deposits, and tax accounts
Verification of all ITCs and ITRs
Validation of supplier tax numbers
Review of POS/e-commerce tax settings
Re-checking GST/QST formulas
Preparing a complete audit-ready support package
Responding immediately to CRA/RQ requests
By removing errors proactively, we prevent refund delays, penalties, and audits.

Mackisen Service Hub
We provide:
Full GST/QST filing services
Monthly/quarterly reconciliation
Digital platform tax audits (Shopify, Square, Stripe)
Tax code optimization
Filing corrections and amendments
Audit defense and documentation preparation
Training for owners and bookkeepers
Our team ensures 100% accuracy every period.

Why Mackisen
With 35+ years of CPA expertise, Mackisen CPA Montreal ensures your GST/QST filings are error-free, compliant, and fully defensible. We protect your refunds, prevent penalties, and safeguard your business from costly filing mistakes.