Well, I live near the customs office and know quite good about how the paperwork shall be done, so it is usually just about a 10-15 minutes visit in the office, VAT is payed if necessary and done. I will always pay more using the GSP, than I could get it through myself.
However as you said, shipping abroad from US is a problem for the sellers. But not always, have worked with some great people from the US, and very bad sellers from EU, especially Italy.