You can add product options to each product.
This is mostly helpful for T-shirt sizes but you can also use it for other things like different water sizes etc.
Each option you can specify a different price. Handy so you can sell your 500 mL waters for $1.50 and your 2 L water for $3.00
The product options will carry over to your pre-order so with that you will know what t-shirt size each person wants.
