As many cover collectors will know, the USPS authorizes numerous pictorial cancellations for various local events at many locations around the country promoting various themes. I
selectively order some of the cancels that look especially nice as its a refreshing change to the spray-on cancels I see in day-to-day mail.
Anyway, according to The Postal Bulletin, all one has to do to request these cancels is to provide a self-addressed stamped envelope or postal card with sufficient first class postage mailed to a special "station" address in the cancellation community and they will apply the cancel and return the cover via the mailstream. If you desire they be returned unaddressed and in a protective cover, you
may (but do not have to) enclose a larger self-addressed envelope for the return of the covers so they do not get undesired markings in the return mail. Traditional FDC's are returned in a plastic sleeve and cardboard backing without charge; some local cancellation stations may include the returned cancels in a plastic sleeve or even a USPS postage paid envelope. That is a kind gesture on the part of the post office to be sure, even though I do not necessarily request it.
My typical way of preparing such covers is to request that they be returned via the regular mailstream and therefore do not enclose a return envelope for the covers. Instead, I place a piece of Scotch 811 removable tape at the bottom margin of my covers which includes a return address label and sufficient space so that the barcode is affixed to the tape, so when they arrive in my mailbox cancelled, I simply remove the tape and I have an unaddressed cancelled cover without the undesired postal markings and save the cost for the extra stamp and envelope. This procedure has worked reasonably well with few problems.
I recently requested this cancellation:

It arrived back to me in a protective plastic sleeve (which I had not requested) along with this note, suggesting that I should have enclosed 20 cents additional postage:

Has anyone ever heard of this "
20 cents additional postage " requirement? ...OR...
Has anyone taken this regulation to bear and enclosed a glassine envelope with 20 cents postage on it for the return of such covers? I suppose that would make an interesting postal history piece, as I can't say that I ever recall seeing 20 cents in postage cover anything except the second ounce of first class mail and I was curious how the USPS arrived at requiring the amount of 20 cents additional postage for this purpose.
I'd be interested to hear of other collector's experiences in this process.