Just use these. Trust me. They are the standard 10x20 tray but FOUR TIMES AS THICK. I have carried an entire tray with 4 " deep square pots filled with a sand/vermiculite mix with one hand. Seriously, just one hand. Don't believe me? Then check out this picture!
Shipping might cost extra on these. Not 100% sure how Shopify will calculate it. I'll let you know if I have to adjust cost.