Your MIL started the drama? What kind of drama? If its just her gossiping about you keeping a gift but not talking to her, I'd ignore her. If she actually contacted you to start a fight or demand you pay her back by letting her visit or something I'd tell her that she's welcome to purchase postage for it via PayPal to have it sent back to her, or you could call the company to ask about having it returned.
I'd lean more toward ignoring anything she says/does unless she actually outright confronts you.