I use Shop IT it's a mystery shop app for all my shops. If you have an iPhone you must touch the screen for a timing but I hear with Androids you can use the volume button for a timing.
I have not done the USPS shop in awhile is there a special app you need to use now? I have always used SHOPIT for all shops.
It is the Mystery Shop-It Timer & Notes. The picture has a green background with a blue box (with a white question mark). I don't recall if there was a charge. If not, they should charge because I don't know how shoppers live without this app. Anyway, you will find it well worth the purchase (if there is a fee), many times over (and you can write off as an expense).
Mobi works with any company that uses the Shopmetrics platform although some companies do a better job than others in terms of setting up the jobs to be entered using the app.