Blue,
As a proud employee of the USPS the reason we do not accpt pre metered postage is for the exact reasons that you stated. The USPS tracks its performance by the date of the postmark or cancellation, so a pre metered piece is considered stale and can not be used unless it is from the actual point of the meter. Believe it or not if someone continues to use the meter improperly, the Postal Service can have that meter revoked.

But to better info, what I do is mail my flies in a crush proof container with a self addressed envelope of the same size. when the Retail Associate at the Post Office tells me how much it is I buy the same amount of postage in stamps for the return trip. Most of the time It will more than cover the postage to get back to me. If you are really worried about having the correct postage you can always use Priority Mail, which is $4.05 but that is really overkill. Most times about $1.11 is sufficient for return postage. That cover up to 4 ounces of weight.
Sorry I wrote so much but I guess postage can be a real problem.

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Focus on the details in life... Just don't get caught focusing!

Big Willy