Add a Checkbox to Opt In to the UK “Gift Aid” Tax-Incentive on Membership Checkout. About the Add On Gift Aid allows individuals who are subject to UK income tax to complete a simple, short declaration that they are a UK taxpayer ( Wikipedia, 2015 ). If you don’t know what Gift Aid is – you probably don’t need this. This Add On allows you to add a checkbox to membership checkout so customers can opt in to Gift Aid. Installation