Interesting. I shop from Monoprice too sometimes, though it's been a while...
Just keep an eye on your account. So long as it's a credit card and not a debit card, you really don't have a lot to worry about. Every major credit card has 100% fraud protection these days. And if you're really worried, just call the bank and explain the situation. They can immediately cancel the card and send you a new one with a different number.
A debit card can be more problematic, especially if it's linked to your 'main' checking account. I have a friend who's debit card was compromised. He was covered by his bank's fraud protection, and everything was ok after a few days. But waking up one morning and not being able to buy groceries because his checking account was overdrawn because someone else went on a shopping spree wasn't a good feeling.
If your credit card company offers it, sign up to have account alerts emailed to you. I use one card to do all of my online shopping. I've got it set up through the bank to email me whenever *any* activity occurs on it. Gives me a confirmation that charges and payments went through properly, and also the peace-of-mind that if the card is compromised I will be alerted and can react quickly.