I see where the problem lies. Although the official Cornflakes booklets stops at the end of March with 10 missions, there were further missions after the booklet was printed. Looking through my own files I find:
Later official U.S. Army Air Force data indicates that there were further missions in April, the last one on 16 April. The final official total for Cornflakes is 20 missions, 320 fake German mail bags, each containing 300 letters for a total of 96,000 pieces of propaganda mail. I should note that looking through several dozen official OSS documents the number of envelopes in a mailbag is listed as 300, 400 and 800. It could be that there is some confusion between the Hungarian mailbags and the German mailbags, there was more than one size of German mailbag, or the envelopes were simply stuffed differently on different missions. As a result, the numbers we give here are all estimates.
So, it is clear that a 9 April letter could have been delivered through the usual air raid methods.