Vat question

(Uk seller) ive recently signed up for VAT, i know this might seem silly but think ive been making a mistake. I didnt realise amazon was taking the 20% vat as "product tax" and paying it to HMRC. So was working out my earnings and deducting 20% from the income i recieved to pay in vat or take away from the vat i had been charged. Basically paying 20% on all items & 20% of my overal earnings.

Have i been making a mistake? Seems like it