Hi,
The first steps when trying to fix some unposted documents in the SOP module would be to run some reconcile to see if GP can 'fix' it by itself in recalculating some totals.. Sometimes it might be the tax calculations that is off just by a few cents.. I can see that you're using AVALARA Tax calc, and that should prompt you to open a case eventually with them too..
The next step which is more invasive, would be to run some CheckLinks against the SOP module (work & history) to have GP fix any inconsistencies between the records. This is more invasive and can be destructive in some cases, so it's highly recommended that you take a full backup of the company data first and run this when nobody is in GP (off-hours), or even best, restore the backup into a TEST company and run it from there. The output report that is produced might help you identify the culprit and provide some hints to prevent further happenings of this issue.
Reconcile: Sales >> Utilities >> Reconcile
You may wanna look also into this : Sales >> Utilities >> Reconcile-Remove Sales Documents
Click on the HELP icon to understand what's all about this process.
CheckLinks: Microsoft Dynamics GP menu >> Maintenance >> Check Links
Select the Sales module and then select all tables related to "Transactions" from left to right.