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. Once selected, you may find extra information in the ad.
If you need to get in touch with the advertiser, you can use the chat that is available in the ad.
You may in worst case have to abort the ad if you wont get in touch with the advertiser.