You have good intentions but you should stop sending those messages to customers IMMEDIATELY.
Remember no feedback is better then bad feedback or good feedback with low DSR scores. By contacting a customer one or two weeks after a sale and asking "How was the transaction.. is there anything I can do for you" you are effectively saying "I care so much about my feedback/DSR's I will do anything to make you happy" and you open the door to them trying to scam or extort money out of you.
I'm not kidding when I say I get feedback on less then 60% of my transactions. It used to bother me at first but now I could care less.
Yeah I am feeling pretty dumb now about sending them out. I thought it would help increase my feedback. Like you said I get less than 60% feedback also and it bugs me but if it opens me up to scammers then it is better to have no feedback. The guy responded and asked where to mail it but gave no mention of him paying the return costs. I decided to offer him $3 for his return expenses but if he leaves negative feedback I want to have record that I offered to pay for the return so that Ebay could remove it. Oh well this was a fairly cheap lesson to learn.
Thanks for the advice.