I just went through the purchase process on the United Pharmacies website, and you are quite right, they do not seem to accept credit cards. The last time I purchased from them was in 2012, and at that time they were accepting credits cards.
Well, you might want to buy the amisulpride from Goldpharma. They have various makes of amisulpride available (see
here), and the one I bought from them just a few months ago was this brand of amisulpride:
Amisulprid-biomo 50mg 50 tbl for £26 ($44). I have used Goldpharma quite a bit, and they appear to me to be a quality pharmacy.