How does a UK-formed LLP get a US merchant account? Do we need a US EIN?

If budget is not your concern, form an off shore company in US.

This is a tricky issue and the rules are getting tigher. Generally, if you use a payment gateway that is non-us (and hence a non-us merchant account) there is a chance that your customers will get charged a foreign fee. To avoid this, you'll need to set up a U.S. merchant account/payment gateway such as paypal. While Paypal does have a global solutions account, the only ones that allow you to not go off the page (API) are U.S. and UK. Your best bet is to set up a U.S. Company which can qualify for a U.S. Merchant account. This will insulate your current business from the worldwide taxation that will result from this. However, you have to have an exit strategy if you don't want the profits to be trapped in your U.S. subsidiary.

I've dealt with a few Canadian companies that do alot of cross-border sales. They all seem to form a subsidiary in the US and have a maildrop on the US side of the border.

Complex issue..with standards tightening due to fraud in this area. We were faced with the same issue. We began in the US and could only accept US based cards primarily. We could not get a API from a European based payment system without a formal company there. As our need grew we created a OY in Finland and finally a PTE LTD in HK . This allowed us to embed three different API's into our code and it is code triggered by the final destination client. It gets a bit complicated as at times we collect EURO but buy goods from US or HK.

I suppose you would have to look at what service you provide and if it is justified on whether or not worth accepting dollars VS local currency. We also had that lovely VAT to deal with. For us it became such a headache we eventually abandoned the CC function and just went with an escrow service but we are not selling to the public, it is business to business.

Hope this helps.