Yes, what she said! You have a right to cancel within 3 days for any reason. You should have a sales receipt that provides that information. If you don't, cancel the check.
Here's my advice on how to help yourself say no. Make a "family policy". Blame it on your dh if you have to. ;) But make a policy that you will not buy anything from door to door salespeople, over the phone, etc. Or if you just really like to talk to these people, make it a policy that you (or your dh) will not buy anything from door-to-door or over phone without consulting the other.
Then you can "blame your policy" rather than have to come up with reasons/excuses. "I'm sorry, but we have a policy of not purchasing these items without consulting the other spouse. I'll be happy to take information on your product." Repeat as often as necessary. Do not try to think of other reasons. Do not give in to the pity party. Just repeat your line as many times as it takes!