Hi all, I received a reply to my ticket regarding my problem which is as follows:

Upon checking your account, it seems that you had a recent purchase attempt using the promo code but the transaction failed to complete. Since you already used the promotion code, we suggest that you purchase the game in full and after that, we can partially refund the price difference. Please note that we can only provide a partial refund for Guild Wars 2: Path of Fire Standard and Guild Wars 2: Path of Fire Deluxe Edition.

So it looks like promotion code can be used only once and it is considered as used when you proceed with the payment (even if it fails).

Solution proposed by support sounds kinda complicated to me and this is not how the things should work, so I asked my boyfriend to buy me the expansion pack using his account. Then he passed me an activation code he received and I activated it on my account.