I would think that for purposes of printing your invoice, it would be most flexible to always enter your purchase amount in the same field regardless of the currency used. Your last method seems to do this.
For a US$ based business, I'd have at least the following fields: Amt, Currency, Conversion Factor, US$ Equivalent
You'd enter the amount and select the currency (you can set it to default to your most common currency). The conversion Factor would be looked up from a reference table of current exchange rates and US$ Equivalent would use the Amt and Conversiont Factor to compute the value in US dollars.
ok... thanks. i think that makes sense. i have a foloow up question with regard to getting conversion rates, i know that there is a way (although i am not too familiar with it) to get information like google maps incorporated into filemaker, but is there a method to get current exchange rates imported into filemaker more or less automatically? for example to get today's euro to dollar rate from the web with a single click or two?