E-Commerce Help!

I am building a large web site for a local company that is not based in the USA or Canada. They need an online donation page where they can accept donation payments. They will probably receive no more than 20 donations a month. Can someone explain to me how the ecommerce set up works.
They would need to open a local merchant account. I could probably work with oscommerce but do i need also something like authorize.net or verisign to process the credit card? Is there some way to do it without these 3rd party gateways?

You'll need to work through authorize.net or similar, unless you setup your own merchant account and process transactions yourself.

oscommerce has plugins for authorize.net and several other online payment systems... you should be ready to roll out of the box.