I have worked with authorize.net, and that is my only experience so far with the process.
You may want to read this article on the subject (Although it refers to .Net the idea is the same).
www.west-wind.com/presentations/aspnetecommerce/aspnetecommerce.asp

In the case of authorize.net, they require you send a URL complete with merchand ID, merchandPassword and the rest of the information on the transaction. Once they receive a properly formatted URL, you get a response on whether the transaction went through or not (if it does not go through, they normally return a reason why it did not go through). In their case they povided sample code in VB script that was easily translatable into Foxpro. I ended up using MSXML2.ServerXMLHTTP to submit the URL. I can only assume that other provider work in a similar fashion. Authorize.net by the way only accepts USA transactions, not canadian, but in our case that was what we needed.


>Dear Mike,
>
>Thank you so much for your guide, I checked http://www.authorize.net/ and as you have mentioned there are a lot more on the Net such as:
>http://www.chargegateway.com/
>http://www.arvicsecure.com/
>http://www.ccavenue.com/content/credit_card_processing.jsp
>http://www.merchantexpress.com/
>http://www.paynetsystems.com/
>http://www.collectivepos.com/
>http://www.richsolutions.com/
>and ...
>
>If you had an idea to present a path to know what to do, I would be appreciated.
>
>For example, there is a POS machine or a sweep card reader, then our application should be connected to the device, I think it should be come with an API, then we need a secured session layer SSL service and a service provider to process the credit cards, am I right?
>
>...Farhad