Can a retailer charge me sales tax after being charged?

You didn't indicate where this took place (U.S. or elsewhere), but at the end of the day, if they're supposed to charge you tax and it was legitimately a systems issue that prevented collection at the time of sale I don't think there's anything improper or illegal about it.

UPDATE: It occurred to me to add the fact that the sale is not technically complete until you receive your merchandise, so for them to charge sales tax upon shipment is not much different from a retailer who doesn't charge you at all until that same point. And then, and ONLY then, is the transaction complete.

That being said, I'd think if you were to write a polite email to customer service protesting this, good public relations might cause them to offer to waive it as a one-time courtesy. It never hurts to ask, right? The answer's "no" if you don't.

Retailers (in the US) do not charge sales tax. The government (or the voters, if you like) charges the tax. The retailers have a legal obligation to collect it from you, and you have a legal obligation to pay it. But as SRiverNet mentioned, the retailer might pay it for you just to be nice if you ask them.