Virginia- No my dress didn't get ruined. I still have it, of course it would need to be dry cleaned which I still haven't done yet either. The Tulum Extreme we did I think was about a $100/person, but include all the activities and lunch. The half day trip I think was only about $40/person and only goes to Tulum. We had booked all our trips through apple vacations so we just booked the trips with our apple rep who is stationed in hotel. If you are booking so type of package they all have people in the lobby to book excurions. You can also go right into town and book excursions. Also, if you want to go to Tulum only with a larger group you could probably just hire a transportation service to take your whole group too.
There is 5th avenue to buy things which is about a 15 min walk or 5 min cab ride like carolyn said, but there is another little place to the left of the hotel. It's about a 5 min walk that you can buy things at too. Walmart is right outside Playa about a 10 min cab ride too.
Jrb-We only had about 54 people, but they had no problem handling it and I'm sure they won't have any problem handling yours either.