Hiding PIN and user name on site

I'm a volunteer for a foundation and we're using ViaKlix to accept donations. I am new to this. We've had someone periodically going to our terminal, presumably from information that they got from the code of our site, and then running through hundreds of fake donations at a time to try to find out what card numbers they have that are valid. ViaKlix won't help me with software issues, but they tell me that I have AVS address and zip enabled, but still we are getting "donations" which don't match the correct information for zip and address. Could someone take a look at the site, and tell me what I need to do to the code, if anything, to make it decline donations where the address doesn't match? And the bigger issue is that our pin and user name for the terminal show up there, and I want to know how to encrypt that. ViaKlix's service rep said that it could be done, but wouldn't say what to do. I tried one thing I found by googling, but I don't think it worked. Here's the site:
https://metzgerl.powweb.com/donatenow.html

It is hard to provide free advise on a forum like this, since it requires taking a thorough look at your implementation. I don't think that someone will take this over free of charge, but perhaps you have luck :-)

My (free ;-) advise is to search for an other payment processor. Obviously ViaKlix (which I don't know by myself) seems not really interested in your business. Support should be a matter of course...

Thank you, I do plan to find another payment processor. ViaKlix's support has been terrible. But we have the contract for a year which ends in August.