My own calculations also support the key takeaway that the only...

My own calculations also support the key takeaway that the only way APT can make any money is through the late fee that they charge.

Really???
This is clearly how they do not make money. Merchant fees - Merchant fees is the model.
If no late fees were charged at all it would be great for APT business model. They need to be there as a deterrent or there would be a lot more bad debt.

Capital for APT becomes cheaper if all customers repaid on time.

I honestly can't see how you can make a case that the business model depends on late fees to succeed.