I will soon have to collect sales tax in more than just my home province, and I'm wondering if there are any best practices in this matter.

I currently have a field for the rate (auto-enter the current Ontario tax rate), and a calculation field for the tax.

Do I go simple - just change the rate depending on the ship-to province, and carefully make sure that sales-tax-reports never combine them?

Or do I go a bit more complicated and add a new field, or (gasp!) a related table for sales tax. (might be an especially good idea if I plan to collect sales tax for a larger number of provinces in the future?)

I've noticed looking at other invoices that it seems to be a common practice to make sales tax a regular line item and sort to the bottom (I base that on its position on the invoice). Wondering if that's the common way to do it?

If there are going to be different sales tax rates for different areas, I would store them in a seperate table. Even if there was ONLY one, I would still store the rate in a preferences table.

I'm pretty happy putting the rate right into the invoice creation script or as an auto-enter calc - it doesn't have to be modifiable by anyone but me. I was wondering about the actual sales tax amount field itself.

I could certainly use one field to calculate the sales tax regardless of the province, as long as I remember that reports need to be designed so as not to present a sum of this field that includes invoices from different provinces - I might inadvertently pay all of my sales tax collected to a single tax authority, for example.

I was just wondering if there were any good arguments in favor of using another method instead, such as putting every sales tax amount in a related record, which appears to be a common practice.