How do I get them to stop?

I just registered for the Amazon Card (don’t know how it works tbh) and I want to be able to see all the information for paying it off and such. It says that I need to link to a synchrony account (of which I don’t have any) so I was wondering how to find that information. It asks for the Account number (of what? Idk) and then a zip code.

Does anyone know how Synchrony Bank applies payments when you have balances among all three of their non promo and promo plans (non promo balance, deferred interest with X equal payments, deferred interest over x months). I called Synchrony Bank to ask and spoke to two different reps, neither of them could explain it or point me to any account terms that would describe the process. They only pointed me to the 'Allocation' of payments screen in your online account and actually said that 'a lot of customers call in to allocate their payments'. It seems to me that Synchrony Bank keeps the details of how their system works undocumented for it's customers with the hope that they mess up the calculation somehow and miss the promo period.

What prompted me to call the bank was because I noticed on my last two statements, they were allocating almost all of the remaining balance of my payment after paying the non promo balance to the deferred interest with x equal payments balances instead of dividing the balance by x months and allocating that amount only, then applying the remaining balance to the deferred interest over x months. So what was happening is the x equal payments balances were being paid off in less time while the deferred interest over x months balances remained untouched.

Example:

$200 non promo balance

$240 deferred interest six equal payments

$1200 deferred interest for 12 months

In the above example, I would pay $200 (non promo) +$40 (240/6) + $100 (1200/12) = total payment of $340.

What Synchrony Bank does with my $340 payment:

Applies the $200 to the non promo balance because I chose pay non promo balances first as my allocation choice on the 'Allocation' page in my account, $140 to the six equal payments, and NOTHING to the deferred interest balance. This was a simple example to show what the bank is doing, but imagine as in my case you have multiple balances in the each of the deferred categories. How do you know what to pay? The Synchrony Bank reps could not tell me, just that I needed to call in each month to correct the allocation, if I thought it was wrong. But what happens if I miscalculate and they apply the balance wrong and I miss a promo period? Have to fight with them each month to allocate payments the correct way?

Am I missing something? Am I looking at this all wrong?