If the pick-up items are outside the house or apartment simply pick up and press "Done". In most cases you don't have to contact the advertiser. However, if you need to access the advertisers premises, check to see if they have shared the entry code in the ad.
If you need to get in touch with the advertiser, begin by calling them or send a text message. This can be done directly from the ad.
You may in worst case have to abort the ad if you wont get in touch with the advertiser.