Xbox Gift Cards are now sold through an "Agency Model". This means that the distributor and reseller never "owns" the product. The product is sold to the end consumer directly by Microsoft and you, the reseller, receive an Agency Fee for handling the transaction.
Exertis will invoice you for the full face value of the card i.e. for a £15 card, we'll invoice you £15. You will in turn charge your customer the full face value. Every 2 days, we will calculate your Agency Fees, raise a Credit Note and credit your account. This should be considered your "income" and you should declare the amount you received for Agency Fees, for tax and VAT purposes.