E commerce credit card question

I saw something on this topic posted long ago, but with technology changing
daily, I would appreciate any feedback.

I have a client who would like to accept 'real time' credit card
transactions.  He is trying to keep costs at a minimum on his multitude of
web sites.

Does anyone know of reputable companies that provide this service?  Do you
know if you can set up just one account and use it on any number of web
sites?

Thanks so much!
Juliana
PS, if anyone has an answer to my previous post about the frames, that would
also be great!

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