Gahhh I did this once. I had just gotten back from a trip to Australia and New Zealand, and I had all my ticket stubs to send it for frequent flier miles. A trip from the East Coast to New Zealand to Cairns to Sydney to the East Coast again? That's a ****load of miles. Unfortunately, the postage rates changed from 32 cents to 37 cents while I was gone, and our postmen are bitter at life here, and I never saw them again. I lost a good 20,000 frequent flier miles for that stupid mistake.
I hope it works out better for you!