Forgot to Charge QST on Some Sales? How to Correct Tax Collection Errors

Many Quebec businesses eventually make the same mistake: they issue invoices without charging QST (or GST/QST), only to realize later that the sales were taxable. This happens to freelancers, consultants, retailers, contractors, e-commerce sellers, startups almost everyone.
The key is correcting the mistake properly to avoid penalties, interest, customer disputes, or a Revenu Québec audit.

This guide explains exactly what to do when you forget to charge QST, how to fix invoices, and how to stay compliant going forward.

Step 1 — Confirm Whether the Sales Were Actually Taxable

Before correcting anything, determine:
• Was the product or service taxable?
• Was the customer located in Quebec?
• Was the sale consumed or delivered in Quebec?
• Were you already required to be registered for QST at the time?

If the sale was:
✓ taxable
✓ made to a Quebec customer
✓ and you were registered (or required to be registered)

→ You must correct the QST error.

Step 2 — Determine the Period Affected

Identify:
• which invoices were missing QST
• which filing periods are impacted
• whether corrections affect current or past FPZ-500-V returns

A CPA can help map the corrections to the right periods.

Step 3 — Decide How to Correct the Invoice

There are two ways to correct missing QST charges:

Option A — Issue a Corrected Invoice

You can send the client:
• a revised invoice including QST
• or a supplementary invoice showing QST only

However, the client is not legally required to pay retroactive tax this is the biggest risk.

Option B — Absorb the QST Yourself

If collecting from the client is impractical:
• you treat the original amount as tax-included
• calculate the embedded QST portion
• remit QST out of your own revenue

Formula for tax-included QST:
QST = Total amount ÷ 1.09975 × 0.09975

Most businesses choose this option to maintain client relationships.

Step 4 — Correct Your GST/QST Return

You may need to:
• amend a past FPZ-500-V
• file an adjustment
• correct QST collected
• adjust ITCs/ITRs if needed

Revenu Québec expects accurate reporting once the mistake is discovered.

Step 5 — Document Every Correction

For audit protection, document:
• original invoices
• corrected invoices (if any)
• tax-included calculations
• correspondence with clients
• revised GST/QST filings
• ledger adjustments

Proper documentation prevents future disputes with Revenu Québec.

Step 6 — Fix the System That Caused the Mistake

Most QST errors happen because:
• accounting software is not configured correctly
• POS or Shopify settings are wrong
• invoice templates don’t show QST
• staff are not trained
• taxable vs. exempt rules are misunderstood

Mackisen ensures the error never repeats.

Step 7 — Monitor Future Invoices Carefully

Going forward:
• verify all tax codes
• automate GST/QST in your software
• review invoices before sending
• reconcile GST/QST monthly
• ensure new products/services are mapped to the correct tax codes

Small errors caught early prevent large reassessments later.

What Happens If You Don’t Correct the Error?

If QST is not corrected:
• you may owe the tax out of pocket
• interest accumulates
• penalties may apply
• Revenu Québec may reassess past periods
• refund claims may be denied
• repeated mistakes increase audit risk

Ignoring the issue makes it worse.

Real Client Stories

A freelancer forgot QST on 20 invoices. Mackisen recalculated tax-included QST, filed adjustments, and protected them from penalties.

A boutique sent invoices without GST/QST for three months. Mackisen corrected all filings and updated the POS system.

A contractor charged GST but not QST. Mackisen rebuilt the tax structure in QuickBooks and prevented a future audit.

Common Questions

Can clients refuse to pay the QST?
Yes and most do. Many businesses must absorb the tax.

Will Revenu Québec penalize me?
If corrected promptly and professionally, penalties can often be avoided.

Do I need to amend past filings?
Yes if the error affects previous reporting periods.

What if this happened for many months?
It can still be fixed reconstruction is common.

Why Mackisen

With more than 35 years of combined CPA experience, Mackisen CPA Montreal helps businesses correct QST mistakes, fix invoices, adjust filings, prevent penalties, and implement systems that ensure tax accuracy moving forward.