You _could_ do a donation like thing by putting a product or a few, which is called “donation” and in different amounts.
Though – my customer would really like a non fixed value for the “product” price for the Donation product. Is that really not possible with eStore? I would like to keep the donation in the same cart than the products that have been chosen – if the customer would like to both donate and buy some goods from the shop, and pay at once.