|Unlock Forum posting with Annual Membership.|
When you add PBS money to your account, there's currently a 50 cent transaction fee. It's a flat fee -- the same if you add 49 cents or a large sum of money. So it's better to add at least $5 or $10 dollars rather than paying the fee each time you need a swap credit.
Alternatively, if you print with PBS postage, you would earn a free "swap-fee". The postage is the cost of the stamps + 55 cents, and includes "instant credit" when you mark the book mailed, and a guaranteed credit if the book is lost in the mail (for members with good sending records.) You can also drop your book in a mailbox instead of taking it to the Post Office, even if it is over 13 ounces. Not bad for 6 cents more. And for light-weight books, the postage rate for packages with electronic postage is less than that for packages with stamps, so it's possible to come out ahead.
If you plan on printing PBS postage, it might be better to add $20 or $30 to your PBS account, since the postage money would come out of that account as well.