Everything is very, very expensive in Punta Cana/Bavaro due to the high level of tourism. You can go to Huigay which is about 45 minutes away and it has 2-3 stores similar to small Walmarts. It cost about $50-$60 for round trip and 2 hour shopping time. The Bavaro/Punta Cana area is not the place to go shopping unless you are looking for tourist "junk" to take back home.
You can go to the "tourist" mall in Bavaro called Plaza Bavaro but it mostly junk, junk and more junk. (t-shirts, beer mugs, tourist clothes, etc.) Make sure that you haggle as their starting price is 3 X's the real purchase price.
I would suggest that you enjoy the beach and relax and wait to go shopping "back" home.