In EShop, there are 2 configuration options which require customer's cart must meet before being able to checkout products. Go to EShop -> System -> Configuration -> Option tab, look for the following options:
Minimum Sub Total: If you enter a number for it, then sub total of the customer's cart must reach before being able to checkout. Otherwise, customer will receive a wanring message when trying to checkout products:
You can use one option or both of them. If not, simply enter 0 for them.