Reasons to offer online payment processing:
1. A convenience to customers/clients/members
2. A way to receive payments sooner
3. Automated/recurring billing
4. Elimination of printing, paper, envelopes, postage
5. To save money and time
Below are some of the available merchant services that can be integrated into your site. There are various models, service levels and fee structures that will ultimately affect which you choose.
Cashie Commerce is the eStore hosting service we recommend. It's easily maintained by you and seamlessly integrated into any Web site. It tracks inventory and processes orders through an easy to use interface.
SitePoint Article regarding gateways