I don't know what the specific issue is, but many sites don't support online calculation of shipping items outside it's country of origin - and many financial institutions don't process (or guarantee) payments from credit cards held in other countries.
Keep a few dollars in a Paypal account, so that vendors can see (if they wish) that you are using verified funds.
Shipping is where sellers struggle, especially if the products vary greatly in size and weight. For example, a container of leather treatment is different than the same-sized container of paint in some countries. For example, BMW Motorcycle Paint has historically not been allowed in the US (chemical hazard) so repairs must be done with alternatives.
Aftermarket pipes are shipped all over the world, but most are done through established distributors or one-on-one verbal negotiations in order to ensure that the seller still makes money on the deal.
One of my clients had me disable the online-shopping features of his site altogether, and tells me that the overall experience has since been more positive because there is no confusion or concerns on with both him and his customers.
Hope this helps,