membermojo includes a gift aid field to record the member's tax-paying status, and entry of an arbitrary donation amount.
Both fields can be added to your form from the Add Field / System defined fields section of the Member Form page and both can be given a new label if the default is not appropriate.
When the Gift aid field is added to your form any payment downloads will include this field and the member's address.
Donation amounts are entered during membership application or renewal. The donation is added to any other fees before payment is taken.
Donations can be viewed and downloaded from the Payment Items page.