I really like this idea! I think this could be a fun and challenging co-op game with just a few minor tweaks to balance. If you are going to reward the player with more time for completing a task, the task has to be finite or the rewarded time has to get progressively smaller. In this case, if you receive +10 seconds for collecting the first item, maybe the next item gives +9.8 seconds, and then +9.6, etc. That way it's impossible to play infinitely. You could also make the number of points based on the time remaining, to guarantee that the fastest players have the highest scores.
The art was very cute, and the idea was very sweet. I love the idea of cooperative shopping. The balance issues are totally reasonable for a jam game, and you have a lot to be proud of in this submission!