I have my taxes set up and have them applied to the appropriate items, but the taxes are still not applying to my invoices. How do I get the taxes to apply to a previously created invoice?

Welcome back @random_writings.

If your Invoice has already been sent out, we can't edit the invoice to add taxes. Taxes on invoices are usually applied once you enter the item under Line Items.

I suggest that you take a look at your tax settings located in your Square Dashboard under Item > Sales Tax > Click on the tax rate that was created > choose the option Add Tax to Item Price.  

Hope this helps!