"trying to navigate the site using my mobile phone."
There are a lot of problems with using mobile apps on ebay. They do not work as well as the full site.
I hope you have placed the non payer on your blocked bidders list.
Never bother asking a non payer to pay. Just start the unpaid item process 4 days after end of sale and close it 4 days later.
If your non payer is like most, he will not pay for another item with another seller and he will eventually get enough strikes to be blocked from bidding.
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Non payers should get unpaid item strikes and be added to block bidders lists. Period.