You kind of didn't highlight:
They promised you what they were promised, and their supplier reneged on the deal by putting you on backorder.
The beauty of drop shipping and "just in time" inventory is when inventory is "just in time", everything flows smoothly. Unfortunately, they're very susceptible interruptions when things go pear shaped. A lesson learned during COVID, everywhere, in all industries, at all levels. We are still reeling from this directly and indirectly. (No, I'm not suggesting that this is COVID related, simply reflecting on the fragility of these systems.)
It's your money and your time. Communication is a challenge at the best of times. They look like one of the million other drop shipping virtual storefronts on the internet trying to eek out a living with 5-10% markup. I'm sure they didn't bargain having a living room gorged beyond the load bearing weight of the floor in ammunition when they decided to start this project. Much less all of the expense, time, labor unpacking, repacking, addressing, checking, all of the orders that they are even more overwhelmed with since they have to do it all themselves.
They signed up for taking orders and forwarding them to the fulfillment company, not this. They are likely reconsidering some life choices right now.
Like I said, it's your money and time. Sounds to me they're owning the problem and coping as best as they can, but certainly get your money back if you don't want the ammo anymore.