We recently fixed a bug in .xls import and export of project information to spreadsheets, and decided that it was time to also support the newer .xlsx format, as .xls is becoming less common (and some apps, like Numbers, no longer support it directly.)
This feature is added from version 13.1, which has just been submitted to Apple and will be released in around a week. I hope you find it helpful.
The first time you export, you will be asked to choose between .xls and .xlsx. If you need to change this later, you can do so from the ShotList page in Settings.
ShotList can still import either type of file, without needing to make a change in Settings.
Soluble Apps 