With many payment gateways, you don't actually need a complete 'shopping cart' script.
It depends on how many items you're selling, etc. but if only a few, a few lines of code will suffice - PLUS the gateway will write the code for you automatically - at least the one I'm a dealer for does. So does Authorizenet.com and most of the others. The only gateway I've had bad experiences with is Bank of America.
Some gateways want bags of money just to set you up ($400+) and some will set you up for $50. I've put up a little info about gateways and merchant accounts at:
http://www.compusolver.com/hosting/faq/
One more thing - will nearly all of these gateways, you do NOT have to have a secure server and you do NOT take the credit card info on YOUR site.
You take the customer and purchase info and hopefully store it in a database before sending the customer off to the payment gateway for credit card info. Then the gateway sends the customer back to your site with info to let you know whether the transaction suceeded, confirm the amount, etc. via form variables (hopefully POST vars).
I hope this helps.