What is the ruling on taking $20 USD as administrative fees when purchasing an item from a company in installments, knowing that its original price is $850 USD, and the installment period is a full year?

There is no harm in increasing the price in installment sales, provided that both parties agree on the payment method before concluding the contract. The twenty dollars cannot be judged except by knowing their true nature; if they are a known fee for services, then there is no harm in them. The correctness of this is supported by the fact that it is a fixed amount and not a percentage.