Redbubble is 100% free to to upload items.
How they make their money is this:
If a t-shirt base price is $17, and your own markup is 25%, you will get $4.25 (25% of $17) from the t-shirt sale, with the retail price being set $21.25 (so your customer pays $21.25, you get $4.25, Redbubble gets $17).