Your math is fault. $10,000 in 42¢ stamps is 23809 stamps (you can’t buy half of a forever stamp.) At 44¢ those stamps would cost $10475.96.
Except that the postage may only go up 1¢; also even if you achieve your desired 4.7596% increase, you do not have an extra $475.96 in your pocket - you have 23809 stamps that you have to stick on envelopes to get your money’s worth.