Customer-specified price in Drupal 7 Commerce

Today, I spent several hours messing about with the Rules UI and googling Commerce docs and forums, trying to figure out how to provide a field where the customer can freely choose the price of a donation-style product. In the end, I found pointers to a method which turns out to work pretty well.

My use case

Donation as a Drupal 7 Commerce product. Donation amount can be chosen freely, above a fixed minimum. The customer/donor receives a reward (but that is not actually relevant here).

Howto

This is how I did it:

1. Install Commerce Customizable Products
2. Add price field
   1. Add a line item type at admin/commerce/config/line-items. It is created with a bunch of default fields, which you cannot change.
   2. Add a field of the Price type (TODO: Figure out how to set a minimum)
3. Make it visible
   1. At the field display settings of your product display content type (something like admin/structure/types/manage/product-display/display), edit the settings for the product reference field. Change Add to Cart line item type to your new line item type. Click Update and Save.
4. Make it count
   1. Add a new rule at admin/config/workflow/rules. Choose the event Calculating the sell price of a product.
   2. Add an Entity has field condition with commerce-line-item for Entity and your price field for Field.
   3. Add a Set the unit price to a specific amount action with commerce-line-item for Line item and something like commerce-line-item:field-donation-price:amount for Amount.