Hello @zayn191 and thanks for your question. Are you talking about adding a small charge or fee on top of the actual amount of money placed on a gift card? I guess you could add a "service charge" for this fee, but in thinking about competing with other retailers, especially this time of year for Holiday shopping, I would reconsider doing this. You may be legally able to justify this time of service charge or fee, but as a consumer when i've purchased gift cards from other retailers I do not recall being charged a fee. You can try it out and see how your customers respond...but my honest guess is your customers may not like this.
Is the reason for doing this to recoup the cost of having to purchase physical plastic gift cards? If that is the case, I would just write this cost of as an advertising or business supply expense on your taxes and this will offset your biz profits. Similar to absorbing the other costs of doing business, such as shopping bags, boxes, advertising expenses, etc.
