WooCommerce Pre Orders 'Add To Cart' Help

I’m looking into purchasing WooCommerce Pre Orders and have been looking at a friend’s copy to demo. It’s a great plugin, but I have an issue I can’t seem to solve.

When ordering, you can only order 1 variation of a product at a time. You can edit the quantity of that variation in the cart but you can’t add 2 variations of the same product to the same cart at the same time. If you do, it will empty the cart and replace it with the current selection. Since I’d be taking pre-orders for screen printed clothing (similar to teespring), ordering multiple variations (sizes in this instance) at one time is important. Making them make multiple orders from the same product would just drive them away.

I don’t want to let customers order from multiple preorders at once since each preordered product has a different release/ship date, but I want to let them order multiple variations, i.e. a Small Tee, a Medium Tee, and a Large Tee, of a particular product since they would all ship at the same time.

I hope all of that made sense.

Here is the code from the Pre Orders Cart file that is responsible for the cart restrictions. Any help is much appreciated.

[excessive code removed – http://codex.ww.wp.xz.cn/Forum_Welcome#Posting_Code ]