We are getting ready to do a job for the local tax office.
Their list has 351 columns with the variable data they want used for different locations within the piece.
I can not for the life of me figure out how to import the list and have access to all 351 columns. I keep only getting 255 columns.
I have tried .xlsx, .txt, and .csv.
Database files won't do more than 255 columns.
Any ideas?

that's a fixed limit, even mentioned in documentation:

"The maximum number of fields supported is 255 for all types of delimited text files. Any column beyond 255 is ignored by PrintShop Mail."

MartinS, thank you.
If I do have the documentation, I have 0 idea where it would even be. It has been at least 5 years sense I installed the software.

You can look at the online documentation. Look at the Delimited Text File Limitations section.

Jean-Cédric, thank you, I will definitely do that.