Commerce Contribute is a very simple module designed to allow users to add a contribution to their order during the checkout process. This module defines a "Contribution" line item type and a checkout pane for configuring contributions.
Configuration
Visit the Store > Configuration > Checkout Settings page (admin/commerce/config/checkout) to configure the contribution pane. It is probably best to place the pane before the Review order pane.
Features
Designation options/funds and minimum donation amount may be specified if needed.