How to Add Taxes and Discounts to an Invoice
Order of operations
The most common source of invoice disputes is not a wrong rate — it is the wrong order. The standard sequence is: total each line, subtract discounts to get a net subtotal, then apply tax to that discounted figure, and finally add any shipping or fees. Applying tax before a discount, or discounting after tax, produces a different total and can over- or under-charge your client.
Settling on one order and applying it consistently means your totals always reconcile and your client can reproduce the math themselves.
Tax: inclusive vs exclusive
Exclusive tax is added on top of your prices: a $100 item at 10% tax becomes $110, with the $10 shown as a separate tax line. This is the default in much of North America, where listed prices are pre-tax. Inclusive tax means the tax is already baked into the price you show: that same $100, tax-inclusive at 10%, contains roughly $9.09 of tax within it. This is common for VAT/GST regions where displayed prices include tax.
Pick the mode your market expects and label the tax line clearly — 'VAT 20%', 'GST 10%', 'Sales tax 8.875%'. The label matters: a client's bookkeeper needs to know what kind of tax they are reclaiming or recording.
Discounts: per-line vs global
A per-line discount reduces a single item — useful when you are marking down one product or writing off part of a specific task. A global discount applies to the whole subtotal, which suits a loyalty discount or a negotiated reduction across the entire order. You can combine them, but be clear about which is which so the client can follow the reduction.
Percentage discounts scale with the order; fixed-amount discounts take off a set sum. Show the discount as its own line rather than silently lowering a price, so the client sees the favor you are doing them.
A worked example
Say you bill two items at $200 and $300, give a 10% global discount, and apply 8% exclusive sales tax. The subtotal is $500. The discount removes $50, leaving $450. Tax at 8% on $450 is $36. The grand total is $486. If you had taxed first, you'd have computed tax on $500 ($40) and reached a different, incorrect figure.
A good generator does this for you and shows each step, so you can hand the client a document whose every line adds up. That transparency is what prevents the back-and-forth.