National Home Gardening Club sends promises and demands more money Hopkins, Minnesota

At first the company looks too good to be true, and i should have stuck with that instinct. I payed the $24.00 for a two year membership upfront and online so i could get $20 in gas rebates a $100 grocery gift card, a pruner and a kneeling cushion. The gas rebates turned out to be in $5 increments that you had to send in 4 different times a year and the grocery gift card you had to mail in, also on your dime, to get a coupon book.
Needless to say at this point i was pretty pissed off. Then low and behold they send me a rebate for the $24 back, saying i won some sort of lottery and i was selected to be a lifetime member! Should have read the fine print.
A few days after mailing in the rebate, i get a book from them in the mail stating if i chose not to send it back i have to pay $20 for it (on the back of a long letter in carefully disguised words) or they will send for collections. At this point i ran to my room found the other letter about the lifetime membership and, also on the back of a long letter in even more carefully disguised words, i found out i now owe them $20.42 each month for 12 months. Over $240.
I checked my account to see how much i have been charged so far, and they haven't charged me anything (unless it was disguised). This makes me wonder what happened to the payment i did online, and why did they respond to it?
Do i have to get rid of the card i have always used for fear of identity theft? How far will they go? Please help!