Odoo allows you, in the inventory app, to configure an option to ask your customers to sign the delivery orders. 

From the front-end Point of Sales, it is actually not possible to use this feature. But, if your client buys something and asks to come back later to pick the product, it is possible to make it work thanks to the "Ship Later" option : 

1. Activate the Ship Later option in your PoS configuration.
2. Enter the order in the point of sale and select your customer. The customer must have a delivery address encoded. 
3. In the payment view, you will be able to select "Ship Later" 
4. Confirm your order.
5. In the back-end, in Inventory app, find your delivery order linked to that PoS order. You are now able to ask your customer to sign from there.