Some of these internet sellers are in less expensive areas to live in, like Florida. They also do not have the overhead that land based shops have. So it costs them less to run their business. Therefore they offer the balls at less cost.
However, most are not that accomadating when problems happen and you are then most likely liable for shipping to and from when problems arise. You also are at the mercy of their shipping schedule. You might get the ball in a week, or two or three weeks and when you get it, it may or may not be what you ordered or the bill might even be different to your online bill(like mine was from a popular online site). Even calling to try to straighten it out, resulting in no returned phone calls etc........ Sending it back is fine, you pay for that, but it still takes time. Time that you certainly would have saved buying locally. It all depends on how much that difference is. How valuable to you is that instant gratification and the ability to warrenty your purchases locally?
In the end, if local shops cannot compete due to land based costs, then there will be fewer places to get your ball drilled. You might even have to go to underground pro shops which really will have not reprocussions or liability. I know most don't look that far into it, but its part of the equation.
Erin