You have actually raised two issues:
- Consumption of inventory items for non-revenue purposes
- Tax-inclusive accounting for sales
The way to adjust inventory without recording a sale is with an inventory write-off. The value of the item removed from inventory will come out at the current average cost, not the retail sales price. See this Guide: Write off inventory | Manager.
Assuming that you also make sales of Ethanol Shield to outside customers, it is appropriate to hold the item in inventory. Or you can hold it in inventory simply as a way of recording the amount on hand so you know when to reorder. But if this is something you use only in small amounts at a time as a consumable shop supply, there is no need to put it into inventory as an inventory item at all. Just create a Consumable supplies expense account and post your purchases of the item there.
Now, on to your question about grand totals and taxes. Receipts and payments are always tax-inclusive. So in your example, you have told the program that you have sold one 2 oz. unit of Ethanol Shield for 6.50, including sales tax. Manager calculates what the price would have been so that when 6.75% is added, the amount comes to 6.50. That amount is 6.50 / 1.0675 = 6.09. Manager posts 6.09 to an income account and the difference, 0.41, to Tax payable. Thus, the receipt shows that a total of 6.50 was received, which includes 6.75% sales tax of 0.41.
Sales invoices, on the other hand are, by default, tax-exclusive, meaning they add the tax amount to the total to give a total balance due. However, they can be marked to be tax-inclusive, just like a receipt.
There are two other things you should know. First, every time you have entered a receipt to record consumption of consumable supplies, you have increased your reported income. This distorts your net profit and will result in paying more income tax. So all of those transactions must be deleted and re-entered as write-offs. Or you will need to back out all such purchases from inventory by posting them instead directly to the Consumable supplies expense account I mentioned earlier.
Second, for every receipt you have entered to record shop usage, the bank or cash account to which you posted that receipt will now have an artificially high balance. The receipt tells Manager your business took in money from the outside. In the case of using inventory as a shop consumable, that did not really happen. So unless you unwind everything as described above, you will never be able to reconcile your bank statement or cash fund.