Howcome ticket harges dont match Quickbooks

We added 3 charges to a ticket;

$11 - Trip
$18.62 - cable
$18.62 - cable
$34.30 - Part
$480.58 - Labor

Quickbooks says they owe $599.72 after taxes, CommitCRM says they owe $563.12 when I hit the summation button.

Why the discrepancy? Each item is taxable in CommitCRM at 6.5%.

If we used the QB link, we'd be in a world of trouble.

First - RangerMSP Tax settings do not affect QB in any way and QB is the only one the decides which taxes will be applied on the invoice.

Second - based on the fact that the total amount in RangerMSP is lower, we suspect that one of the Items you use for the related Charges is not set correctly with taxes and therefore taxes are not applied to it. Please review ALL related Items and verify that they have the correct tax settings.

How can we get a global tax rate applied to all items?

It currently does not work in this way. You should define the taxes that will apply on Items and then define the taxes that are applied on customer accounts. Click here for more.
You can also configure default tax settings for new Items and new Accounts. This is achieved using the Tools > Options Window.

Default tax for items is already set. Default tax for accounts is already set. Where are the settings for everything else that needs to be set?

There are hundreds of items with no tax assigned. How do we batch edit them?

Defaults are set for new records, not existing ones. Unfortunately there isn't a work around for Items or any batch operation, however you can batch-tax-update accounts. Please take a backup (as this is a batch operation...) and run Accounts > Tools > Batch Update Tax Settings

Hope this helps.