There are plenty of POS systems to accept bitcoin/lightning. The issue I find is automatically connecting that to the back end accounting and inventory systems. That seems to be mostly a manual process for now...

If you have to do it manually, its easy to calc, its just a pain to track. From an accounting perspective, you just treat the bitcoin earnings similar as you would cash. Just convert to fiat value (as of the time of sale) when calculating your receipts.

Whatever spot price bitcoin is at the time you recieve is your basis (aquisition price) for that Bitcoin. Most wallets will have a receipt with a $ translation so you can just use that.

If you sell your bitcoin later for fiat you have an additional gain/loss of whatever the current price minus acquisition price is. If you never sell, then there's nothing more to that, enjoy your earned generational wealth.