Well lets see if we can use some logic to solve this without you actually going to the post office to find out shall we?
According to MLB official rules an official ball must weigh no less then five ounces and no more than five and a quarter ounces.
No we go to usps.com and click on calculate postage. This will have to go as a package and not a letter because of the shape, we click "package" and enter a weight of 7 ounces (five plus the envelopes and letter should be no more than seven). It will ask you for zip codes to and from. Lets use to zip codes that are really far apart for fun, like 94070 and 10001 and hit enter. Up pops list of ways to send and prices. Hmmm,...lets see first class is the best way to go and it will cost you $2.19 OR 6 first class stamps for both envelopes.
So there you have it. the answer is SIX first class stamps per envelope will get a MLB ball from one end of the coast to the other and back.
Thanks for your help. I haven't had time to go to the post office because of school. Thanks again.