Can eGifts be stacked?

Yes, eGifts can be stacked.

For example, if your shop comes to a total of £50, and you have 3 eGifts: £10, £15 and £25, they can all be stacked, one after another to pay for the total bill.

The same applies if your shop comes to a total of £50, and 2 eGifts: £48.75, £100. You can use the smaller £48.75 to cover most of it, and finish paying with the £100 so that you're left with £98.75 on that eGift for your next shop.