Never seen that before but how would that help in this situation? Not too sure what all that means anyway?
Not implying that this would help (I don’t know of UK VAT laws, btw) or further complicate things. But if you want to use SCM to associate a sales to a customer, it seems the amount paid PayPal reports in the callback notice will be compared with the sales price stored inside the SCM. And as mentioned, the check allows a difference of 10% if I am right (Derek might know better). So, you might have to keep in mind that adding or substracting a 17.5% VAT, the difference might get bigger than expected by the SCM and the sales might not be processed as intended.
Have a look at around line 628 in mod.simple_commerce.php and you’ll see what I refer to.
Perhaps, best would be to let the customer first choose this location and store two sets of item repositories (one with, one without VAT) in the SCM - and tell the customer that he is required to select the right country. Moreover, isn’t there a further complication were companies (business customers) can give their VAT-ID and aren’t forced to pay VAT even if they are located in the EU?