A circular problem with resolving a sales price

Hi, I am new here but have a maths problem that is causing me to loose my hair lol. I am trying to arrive at a price to sell something on eBay, taking into account the variable costs that eBay and PayPal charges and factoring them into the selling price. However, the selling price also changes too and so it alters the costs. Here is the problem I have:

Selling price $12

+ Insertion fee $0.50

+ PayPal fixed fee $0.30

+ eBay variable fee $x = 5.25% of a

+ PayPal variable fee $y = 2.4% of b

= selling price $a

+ postage $8.50

= total price $b

Such that:

b = Insertion fee + PayPal fixed fee + eBay variable fee + PayPal variable fee + postage.

Note that y affects a, which affects b again, which affects y, which affects a, etc. Likewise x affects a, which affects x again.

Any help would be greatful. Thanks.