The difficulty is that there are so many choices of third party plugins to choose from.

The built-in PoS app is likely the least complicated to implement.

https://docs.btcpayserver.org/Apps/#point-of-sale-app

For your accounting needs, there is the Greenfield REST api which would allow you to automate accounting requirements.

Here is an article that I wrote that may get you going in the right direction.

https://highlighter.com/a/naddr1qvzqqqr4gupzqv9enyt2zcqyug7f8nhjp5r0skt5ayzrl6ytcxmug5q03v7vdcl7qy2hwumn8ghj7un9d3shjtnyv9kh2uewd9hj7qqgv3skxcfnx5enswxhzsq