No, there's nothing to stop him from overchanging you in the first place (same with ordering over the phone or through the mail), but if you're overcharged you call your credit card company and dispute the charge. Noone can stay in business very long doing that, because if the credit card companies keep getting complaints, they'll pull his account and he won't be able to accept credit cards any more.
If you're really concerned, I'm sure you could arrange to pay by postal money order. That won't stop someone from taking your money and not sending what you ordered, but they can't get more than what you send, and it becomes a federal mail fraud case if they rip you off.
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Larry D. Fishel
Side effects may include paralysis
and death but are generally mild.