Don't worry, calculating subtotals before sales tax can be tricky, but it's definitely doable in Excel. Here's how you can do it:

1. First, you need to know the sales tax rate for your invoices. Let's say it's 8.75%, like in your example.
   
2. Next, you need to divide the final total by 1 plus the sales tax rate. This will give you the subtotal before sales tax. In your example, the calculation would be:
   
   
   Formula:

   = $275.14/(1+8.75%) 

   This will give you the result of $253.00, which is the subtotal before sales tax.
   
3. To make this calculation easier for your entire year's worth of invoices, you can use a formula that references the final total and the sales tax rate. Assuming your final total is in column A and your sales tax rate is in column B, you can use the following formula in column C to calculate the subtotal before sales tax:
   
   
   Formula:

   = A2/(1+B2) 

   This formula assumes that your data starts in row 2. You can adjust the formula as needed for your specific data range.
   
4. Finally, you can format the subtotal column as currency to make it easier to read.
   

That's it! With this formula, you'll be able to quickly calculate subtotals before sales tax for all of your invoices.