Here are some tips to avoid this situation.
First, traveling merchants always appear at the same time of year so you can prepare your supplies in advance. Start preparing in winter.
Second, place warehouses near the trading post and set minimum limits on resources for sale. The trading post is supplied only by your merchants, so having a warehouse nearby will significantly speed up the process, and laborers will deliver goods closer to your trading post from far away.
By the way there is a button to cancel task. So you can cancel all tasks except the one with higher priority.