Hi Ryan,
Tax group jurisdictions (GST, PST, etc) can optionally be defined with a minimum tax, so that if the total calculated amount of tax is less than the minimum, then 0.00 tax is actually used for the item. I suspect that this is what's happening, with a minimum tax of 5.00 (or 5% of 100.00). To check this, edit your Tax table and look at the GST jurisdiction tab. If you see a minimum tax not equal to zero, then it can be changed to zero so that GST gets calculated regardless of how little the amount.
Tax table changes cannot be saved when the tax reports have information or there are open batches. If you need to make a change to a tax table and the program won't let you save the change, you have to archive all open batches amd print/purge the tax reports. You may want assistance from your Adagio consultant before doing these things.
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Regards,
Softrak Tech Support