Even making it look easy to order will have a dramatic effect. Not by omitting any information requested previously, but by laying the form out in an attractive open way that made it seem simpler. Make sure the order form explains in full, though with no unnecessary words, what you are offering and what people are committed to. That way, even if they've mislaid the original letter, they can see immediately what they were interested in. Put in a reply-paid envelope .
Nobody likes to pay unless they have to.
And nobody likes to go looking for a stamp, let alone an envelope. If you want to get the most possible responses, have a real stamp on the envelope. Have the name and address of the person you are writing to already filled in, in the order form. It's the equivalent of the waiter serving you as opposed to self-service. More to the point, it should increase your results by a good 15 per cent.