Based off of https://www.paidmembershipspro.com/custom-tax-structure-using-the-pmpro_tax-hook/ to add a flat tax rate to all purchases. Make sure to change tax rate before use.

pmpro_flat_tax_rate.php

<?php
/*
	Custom Tax Example.
	- Requires PMPro 1.3.13 or higher.
	- Leave the tax fields blank in the payment settings.
	- We update the price description to include the tax amount.
*/
function my_pmpro_tax($tax, $values, $order)
{	
	$tax = round((float)$values[price] * 0.055, 2);		
	return $tax;
}
add_filter("pmpro_tax", "my_pmpro_tax", 10, 3);

function my_pmpro_level_cost_text($cost, $level)
{
	$cost .= " Customers will be charged 5.5% tax.";
	return $cost;
}
add_filter("pmpro_level_cost_text", "my_pmpro_level_cost_text", 10, 2);