@WitsEnd2018 - firstly you need to recognise that, despite your own feelings, you did the right thing allowing him to use the loo, and he will have appreciated it, because not everyone would have said yes, and he might have had to hang on, in some discomfort, for a long time, until he could get to a public loo. Plus delivery drivers are on tight schedules, so it can be really hard for them to find the time for loo breaks in their long days.
Secondly, you need to take a deep breath or two, and try to realise that it is not as bad as you are thinking it is. The smell will disappear, and you have cleaned the loo - and the rest of the area is not 'contaminated', I promise you.
This has turned into something huge in your mind, and you have the power to diminish it again by telling yourself that it isn't as big a deal as you think it is, and you have done everything necessary to cope with it. Sit down somewhere comfortable, close your eyes, and focus on your breathing - breath slowly and comfortably, and simply be aware of the sensations, and let your breathing calm your anxiety (because this does sound like anxiety to me). Picture your stress about this as a dark cloud, and then picture the cloud lightening at the edges, getting lighter and lighter, further into the cloud, until it is all gone. I found this technique really helped me with my anxiety and depression, and it might help you too.