pull down to refresh

If a merchant only accepts onchain, there is not much to do when the mempool is clogged but to suffer.
Outside of answering your specific question:
  • Find a merchant that uses lightning, convince your current merchant to use lighting, etc. Find a way towards Lightning. It probably helps if you make it explicit to that merchant that you are moving your business elsewhere because he doesn't accept lightning.
  • Be wise and get your UTXOs nicely arranged when fees are low.
  • Be wise and get your Lightning ready when fees are low.
As you see, it's mostly about moving to lightning and getting ready when the mempool is friendly.