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.
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:
As you see, it's mostly about moving to lightning and getting ready when the mempool is friendly.