In a negotiation it is common to grant a discount on one line and then throw in something small to close the deal — an accessory included, an extra service free of charge. Taken separately they look like minor concessions; added together, they can substantially change how profitable the job is.
Why adding them up in your head does not work
A 5% discount and a free item of apparently modest cost do not add together linearly against the margin: both reduce the selling price while the cost stays where it was, so their combined effect on the percentage margin is sharper than adding the two percentages suggests.
A free item is not a discount
A discount reduces the price of a line that is still being sold. A free item takes that line's price to zero while the business still carries its full cost: it is a heavier concession than a discount of equivalent value, because it wipes out the margin on that line completely.
Keep the total effect in view
The most effective practice is to keep the cumulative effect of every commercial concession — line discounts, final discount, free items — on the job's overall margin visible while you negotiate, not just on the individual line. Only then can you decide knowingly how far to go before the job stops being worth taking.