If you sell a number of products on
your site (or even a few,) then using an Add to Cart button
something like this:
Qty:
will make things very much easier on
your customers.
Personally, I like the PayPal
shopping carts. They're quick and easy to setup on your site. Here's
an example of one I set up on the site www.apathtowholeness.com
Then again, some clients like to use
an offline credit card machine in their office. This can be
done with a shopping cart, too, although it requires setting up
secure CGI software on your site so that all the information
collected will be available for you.