Hi, Topmarket is situated in the shopping area at the Marina. I think you're staying in Cap D'Artrutx aren't you? So from here if you follow road into CnB you will eventually see Paradise Club on your left and rear of the Marina to right with a few car hire places, pharmacy, and the back of Maribel gift shop. You can access Topmarket (sorry yes it's a supermarket) from either this side or from the marina side just to the rear of the Bandstand. Think they needed a couple of days notice (it was my Father in Law that ordered it so not 100% sure) They just got him to write down what he wanted written on it. We definately had candles on the cake, which I assume they provided, might be an idea to pop a pack in your case though just to be sure.
Hope you manage to get something sorted, I'm sure if you asked at the reception of your apartments they'd be able to recommend somewhere also.
With regards the weather...try not to look at the long range forecast, it's rarely accurate (though I always do and know I shouldn't!). I'm sure if it does rain though it won't last long...we had a couple of showers in August but it didn't last long and the sun was soon out afterwards.