Yep, I love this service! The PO does Rock at time..LOL!
Be careful that the "flat rate" that you are using DOES actually say "Flat Rate" on it! I have had several people use the wrong type envelope for swaps. Luckily my PO workers are normal sane people and they help me "cut" the prepaid postage off and put it on the correct envelope. But its easy to get confused on these.