Was there not a record of the orders received and processed?
Or do we otherwise presume stuff ...in other words these orders ...regardless of whoever mailed them...one to Chicago and the other to an LA California supply company...were mailed [very likely] on the same exact day...presumably received on the same exact day...presumably filled on the same exact day....presumably shipped the same exact day....presumably arrived in Dallas the same exact day...and were picked up at the same exact time?
Seaport didn't save a microfilmed copy of the postmarked envelope like Klein's did. So we can only guess as to when Oswald mailed the Seaport form. But since we know the Seaport form got to L.A. on the exact same day as the Klein's order (March 13) and was shipped the exact same day as the Klein's order (March 20), it's probably a good guess to say that he dropped both order forms in a mailbox on March 12th.
You forgot to add that the shipments and/or notice to acquire arrived on the same day too.
we can only guess....it's probably a good guess

I would love to see an actuary calculate the odds of such a series of occurrences randomly happen as mentioned above... I believe it would be just totally unbelievably astrofriggen-nomical.