As mentioned by my colleague, SIN refers to Standard Imports, which is the same tax code you're currently using. So you're on the right track there.

To better understand why the VAT is not calculated correctly, could you please share the total amount on the invoice and the taxable amount that QuickBooks is using to calculate the VAT? It will help us determine whether the 15% rate is being applied to the correct base amount or if there is a discrepancy in how the system is interpreting the figures.

Also, please note that when a VAT amount is manually specified during the importation process (for example, if your importer provides a set VAT value), QuickBooks Online will override its automatic calculation. It is designed to accommodate rounding differences or adjustments from third-party systems.

In the meantime, you can check out this article to see the frequently asked questions about importing data: Common questions about importing data to QuickBooks Online.

The Community team will be on the lookout for your response. We'll gladly help you.