You can set up a product which can act as a variable-price amount, ideal for accepting donations, as follows:
1. Create a product, with a price of 1.00
2. Set its tax-class to --None--
3. If you wish to allow "cents", set its "units" to 0.01 (instead of the default of 1)
4. In the description, be sure to invite the customer to use the "quantity" box to select the price they wish to pay/donate.
5. Fill in all other applicable fields (be sure to set a large number for Products Quantity, since it will still be treated as an inventory item, reducing the quantity-on-hand with each donation made).
Save the product. It is now available in your store.
6. If you wish to allow "cents", then go to your store Admin->Configuration->Stock->Product Quantity Decimals and set it to 2 (instead of the default of 0).