What I paid for the Group Order (GO) ring:

• $373.50+$120 ‎ = $493.50 (GO ring subtotal after GO discount + lab diamond accent upgrade)
• $493.50*0.05‎ = $24.68 (tax)
• $493.50+$24.68‎ = $518.18 (total + tax)

Semi-mount price:

• $749*0.75‎ = $561.75 (GO semi-mount subtotal after GO discount)
• $561.75*0.05‎ = $28.09 (tax)
• $561.75+28.09‎ = $589.84 (total + tax)

What I paid for the stone:

• $493.50*0.05‎ = $24.68 (tax)
• $493.50+$24.68‎ = $518.18 (total + tax)

Price difference calculated:

• $518.18+$518.18‎ = $1,036.36 (what I paid for = GO ring + stone + tax)
• $518.18+$589.84‎ = $1,108.02 (what I should have paid for the GO semi-mount + stone + tax)
• $1108.02-$1036.35‎ = $71.67 (price difference I owe)