First Class package postage went up on October 3 ("holiday surcharge") and will go up again on January 22. If you purchasing postage labels through eBay, but you are getting "weird refunds" or "have to pay more," the only reasons I can think of are (1) you aren't paying for the correct package weight or (2) your shipping label setup page has the wrong ZIP Code in the "Shipping from" section. The latter issue would cause you to often be paying postage based on the wrong Zone.
Could you give more details about a situation where "I have to pay more for first class with my postal service"? Details like package weight and dimensions, both ZIP Codes, where you purchased postage (online through eBay?), what you originally paid for postage and the exact amount you had to pay later.
