How do I control how long items will stay in a shopping cart before a customer is ready to place their order in sitelink?

Currently, if a customer adds items to their shopping cart, but then exits their browser, the items in their cart are emptied automatically. Even if you leave the window up with items in the shopping cart, after 20 minutes, the cart is automatically emptied.

We are new users of MOM and Site Link. Right now our site link check out has a calendar for customers to choose their preferred ship date, which is also linked to the order hold date. The customer submits their order, but the order is not placed until the hold date/preferred ship date arrives in the future. Since the order has technically not been placed or processed, the payment has not been received. This creates so many problems on our end which I will save you some time and not list at the moment.

Our products are on a first-come-first-served-prepay basis, and the customer can choose when they would like their items shipped. Is there a way to process an order/payment the day a customer places their order, and only hold the shipping until the preferred ship date?

Thanks for your help!