First of all, it is illegal in the US to refuse condoms (or anything other than known legally restricted items like booze, tobacco products, or porn) to anyone because of age. Your corner convienence store manager is not in a position to discriminate based on his own sense of morality.
Ok. All condoms are basically the same, the difference really being lubed or unlubed, and receptacle end or plain end. it's a matter of preference. All condoms are tested before they leave the factory (on a machine, silly!). There are directions in the box. You should not keep condoms in excessively warm places as it weakens the latex. And one size fits all.
Although I personally think 13 is too young for sex, and I think that if you're too embarassed to buy them from a total stranger, then you're also probably too young to have sex. Think about that, if you cannot tolerate a little anxiety buying condoms (with all your clothes on), then there's a good chance that same anxiety will interfere with your ability to perform, and enjoy sex. So much for the rant.
That said, remember that the counter person at the local CVS or supermarket is working 8 hours, passing many hundreds of items over that scanner thingy. Do you really think they're going to stop and notice that one box of condoms? nah. And if they do, so what?
If you're going to have sex, regardless of your age (or the anxiety), always wear a condom.