You can set your products as pre-orders in Simple Store to collect payments for products that currently can't be shipped. This is a setting for individual products.
To activate the pre-order option for your product, go to Store > Products. Then edit one of your products for pre-orders. Tick the “Show as pre-order and set shipping date.” option, then set an estimated delivery date below and hit "Save".
Once you publish your site, the BUY NOW button will be changed to “Pre-order” for that one specific product. Your set estimated delivery date will also be displayed in your product description.