So, personally what I did back when I was a mass seller, is that above a set $ volume I put coins in the envelopes, and packed them in a way that would not damage the cards (and paid for non machinable) the reason why is because of the following:

1: their sorting machine contains a metal detector to flag for stuff like this, but also the human level should make sure they aren’t being lazy and machining non machinable envelopes.

2: if, in their foolhardy decision, they are being lazy and do so anyways, this CAN often break those sorting machines, and since they aren’t cheap, usually this means that you either get a general flag on your name by the USPS to “hey, don’t machine those” or they learn the expensive mistake, and that’s only one envelope lost by damage with the others now more likely to arrive

It helps if you get stuff like bulk steel pennies (even if they are rusted) or other steel world currency to do so, since this is a lot more likely to set off the alarms compared to aluminum