Here is what worked for me (and I share everyone's frustrations!) -- create separate orders for items that use flat rate envelop/boxes from those orders wherein you are using your own package. For some known reason, when I did one order with flat rate envelops chosen, separate from the pacakge for which i was using my own box, the payment went through no problem. Why? The world will never know, but hopefully this works for others and thank you to someone suggesting this about using flat rates because that's what led to this epiphany.
Here is what worked for me (and I share everyone's frustrations!) -- create separate orders for items that use flat rate envelop/boxes from those orders wherein you are using your own package. For some known reason, when I did one order with flat rate envelops chosen, separate from the pacakge for which i was using my own box, the payment went through no problem. Why? The world will never know, but hopefully this works for others and thank you to someone suggesting this about using flat rates because that's what led to this epiphany.