Let's get everything set up, dekeusterraf-icl.
We can set up a sales tax with the 6% rate for your government. To do this, you can follow the detailed steps in this article on how to add tax rates: How do I set up sales GST/VAT rates and use them on forms? (International QBO).
After, we can use it when you create the invoice for your customer. With that said, make sure you select Inclusive of tax so your customer still have 50 for the amount to pay. Here's a link for reference: Create invoices in QuickBooks Online.
From there, you can already pay the sales tax at the end of the month: File and pay Tax/GST.
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Feel free to get back to us if you have additional questions on how you can set up everything in QuickBooks Online.