Why am I not allowed to set SPACEGOATS a fixed price but only a flexible RRP for the sales price of my products to Amazon end customers?

The non-binding pricing is in the interest of all contracting parties since price-fixing violates German and European antitrust law. Both SPACEGOATS and you would commit an infringement in this respect if you tried to enforce a fixed price.
The purchase price of your products between SPACEGOATS and you, on the other hand, is specified in Annex 1 to the agreement, as per §5 Par. 1 of the GTC. You will then receive the desired purchase price for your products.