Absolutely scam artists. Let you book a time slot on the website, but don’t actually book the time slot. They will send you a link hours later and expect you to then book a time slot. They obviously expect you to be constantly checking your Viator account whilst on holiday. I had this happen to me and the tour was sold out. I complained and was told no refunds under any circumstances. So I have paid the I travel to the tour location. Have paid roaming charges to complain and also for a tour that I didn’t get, and won’t get refunded for as it was sold out. They will put all the emphasis on you and will say it’s your fault for not booking a time slot sooner. Absolutely scam artists