the issue isn't the canceling it's the way the give you the free postage (you have to buy more).
"$25 Postage Offer:
Yes, you can get $25 in free postage from Stamps.com; that is, with a giant catch included. Read the fine print and you will learn that only $5 of this supposed $25 postage offer is available as implied in the promotional free trial membership. In order to obtain the remaining $20 you must stay a paid member for two months after your trial period is over to be eligible to receive $10 postage credit for each of those months."