Credit card problems shopping online

[Explorer-8]

Recently I tried to buy items online using my credit card. Near the end of the transaction, the site sends me to either paypal.co.uk or nationwide visa activation confirmation. EVERY occasion, it says that my password is wrong even though I write it clearly in red ink, case sensitive, every time I have this trouble. I am sick of choosing a new password and being unable to purchase anything. When I try using the original password, it says something like "password already recognized", but it doesn't work. I wish that items were available in the local shops like they used to be. Please help.