Personally, I found Click Cart Professional to be cost effective (under $100), easy to install, and extremely flexible when it comes to setting it up with different payment services. I'm using it on a couple of my sites, and am just now getting ready to set up a client with a copy.
Functions? Click Cart Pro has them! Calculate shipping costs with a direct link to UPS and USPS; compute sales taxes by region, if applicable; inventory control (keeps track of sold items and removes product from offering when inventory reaches 0); product to multiple category relationships; category to category relationships; automatic download of files sold to customer; modules included to accept payment via numerous sources, including PayPal, Verisign, Authorize.net, iTransact, Planet Payment, 2Checkout, Wells Fargo, and many more; Zero-balance ordering; accept orders for offline credit card processing, check payment, and purchase orders; activate and deactivate product, category or store-wide discounts at the click of a button; printable online order invoices; customizable billing fields...
Okay, I'm getting tired of typing here. Just swing on over to their site at clickcartpro.com and check it out. I actually wasted thousands of dollars purchasing 2nd rate shopping cart systems before I finally came across this one. IMHO, it's got to be one of the best available in the market today.
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