After logging into the webshop you will see the number of points that you still have at your disposal in the top right of the screen. For each gift in the webshop it is stated how many points this item costs. So you can choose a number of gifts for the credit you have.
When logging in, it is stated how many gifts you can choose for your budget. Every time you place a gift in your gift basket, it will show how many points you have left. You then have the choice to donate it or continue shopping.