Recently I accepted two requests but failed to mail them. This was because my books are in my garage and during the period of time I had to mail them the temperature in the garage varied from 15 to 25 degrees.I am 85 years old and my health is not all that good, so my nurse-type wife absolutely forbade me to go out and retrieve them. I can't find any way  to explain this to those at PBS incharge of accounts. 

Use the 'contact us' button at the bottom of the page under the 'Company' heading. Just explain to them what you said here. You coud even just copy&post the note you wrote here.

Hope this works out quickly for you.

Hope this situation was resolved, John.

You can always send a message to your requestor and ask them if it's ok to mail just after the mail by date.

If they say yes, then mark the book as mailed before the "mail by date" and then mail on the date promised (and accepted) by your requestor.