• Pick a niche within gym supplements
• Start selling and experiment a lot - learn your customers’ preferences in terms of pricing, place of purchase, brand, etc.
• Talk to as many customers as you can

With the limited info you have shared, it’s very hard to recommend which product to prioritize. But these should be your guiding principles:

• Prioritize the product that is more scalable (more volume)
• Prioritize the product that will give you greater marketshare in the respective market
• Prioritize the product that is more aligned with your brand & its mission
• If the two products are related:
  • If a particular product is a gateway to other products, offer that at a discount. e.g., if you sell garden pots and fertilizer, think of selling the pot at a discount since it will probably be a one time purchase while markup the fertilizer since it may be a recurring purchase
  • Bundling is also an option

Also, think of today’s profit margins as something that is true today. It can always change due changes in raw material costs, supply chains, your pricing power

Hope this helps. Good luck!

Good job with the website overall. There is scope for SEO improvements (on-page, off-page, & technical). Best luck on the venture