It get's even more confusing when it comes to Yahoo unfortunately. To somewhat answer your question, yes - you can use the services we offer in conjunction with Yahoo.
Basically to process credit cards, you need a merchant account (through a merchant account provider) and an electronic payment gateway (LinkPoint, Authorize.net, Payflow, Yahoo). What you get from Yahoo when you sign up with them is the gateway, hosting and a few other things as well. You can see here
The merchant account provider that Yahoo partner's with is Chase / Paymentech. They use the First Data platform to process transaction, so any processor that is compatible with the First Data platform (like us since we are owned by First Data) will work on their gateway.
has three plans and it seems they raised their rates some (.75%, 1.0%, and 1.5%). This electronic payment gateway fee is applied to your transactions as well as fees imposed by the credit card processor (Chase / Paymentech or First Data / Cardservice). Most gateways will charge a monthly fee and some might charge a transaction rate (fixed fee) while a lot will not even charge a transaction fee. These transaction fees are usually imposed by the merchant account processor.