Shopify can be used as a donor-friendly alternative to Paypal to accept online credit card donations, but it has been a challenge to accept custom/variable amounts where the donor types in the desired dollar amount rather than selects from a menu of fixed amounts. But it is possible to do so with a Cantucky Can-Do attitude.
The lowest price Shopify Lite plan is only USD $9/month + 2.9%/sales + 30¢/transaction and can work well as a payment processor for entities that have a website or use Facebook.
In some cases, an off-the-shelf donation system will better serve your organization’s needs (donation volume, record-keeping requirements, communication channels). I will investigate all options for you to find a method that works well for both you and your donors.