Look at getting your own merchant account through authorize.net or one of the others. I use Bank of America in conjunction with Authorize and it has the lowest transaction fee I could find, .99%. Paypal is something like 3.xx% per transaction.
I use my Authorize acct. with SAP so I can't really offer any solutions for integrating a merchant account directly into a website. All I had to do was create a link (toolbar button) to my SAP Client portal on my website and all was good.