The Post Office voucher can only be redeemed in a singular transaction.
For example: If you have been issued a £90 voucher, you can split the balance for a £45 electricity top-up and £45 gas top-up within the same transaction.
You cannot top-up with £45 on your first store visit and then return to the store at a later date and redeem the remaining balance as the voucher will no longer be valid.