No, and it is worth saying why rather than just no. Opening a pack draws one specific card and takes it out of the pool — nobody else can draw it after you did. That cannot be reversed, so an opened pack is final.
But you are not stuck with the card. For the first 5 minutes after you open it we buy it back at 90% of the value it was drawn at, paid to your Balance, or 100% taken as Harmony Cash, instantly. Take that and you have lost nothing but the difference between what the pack cost and what the card is worth — which is a number you could see before you paid.
An account’s first pack, if it costs $100 or less, comes back whole in those minutes: the card at its whole value to your Balance and the rest of what the pack cost in Harmony Cash, or the whole price in Harmony Cash. Once per person, with the account’s mobile number verified by a texted code before the pack is opened.
After that the card sits in your vault and it is yours. We buy cards back from the vault selectively: when we want one, we make you a Harmony offer on it, in Harmony Cash, for a limited time. Any day you can also list it on the Market at your own price, let us sell it for you, or have it sent home. So "I do not like this card" always has an answer, it is just not a refund.
An unopened pack is a different thing and can be refunded — nothing has been drawn.
The exception is ours, not yours: if a card arrives with the slab cracked, or does not match the certification and grade we published, that is our mistake and we refund it in full including shipping.