No worries! It is quite common to realise you have more stuff to ad once you created your ad. Simply place another ad and select the same helping user who is already on the way if you receive a request from that person, or select someone else if the first helping user do not have the opportunity or, for example, no longer has space left in the car or trailer.
It is not permitted to add more items to the original ad if you have already received a request or selected someone. The reason is that the ad would not be what the person who requested to help you has applied for, which can create problems when picking up as they have expected something else based on the ad's pictures and description that they first applied for.
If you have not received a suggestion or chosen a helping user, you can always update your existing ad with pictures, description or increase the payment by clicking "Change" in the ad, if you want to get more requests.