@Krista371 It could be that they haven't looked at their Airbnb messages, so you could text them on their phone number, perhaps they are driving elsewhere after visiting you. But always use the Airbnb message system as well so that Airbnb also has a record of your attempts to return them.
Message again and offer to mail the boots, have them pay in advance. If the boots are expensive and/or fit really well it could be worth the cost to them, wait another three days for a reply. If you still get no reply, give them a deadline, say a week hence, then donate them to the thrift shop.
I occasionally have guests leave odd socks or other small worthless items so I keep them a few days incase they message me, then throw them out.
One lady left a box of contact lenses, so I did what I suggested you do about the boots but she never replied so I threw them away - I think I kept them for a month because I wear glasses and prescription lenses are very expensive!