A church buying ten copies of a book for a group should pay less each than somebody buying one. A bulk discount is a list of quantities and the percentage off at each - buy 5 and save 10%, buy 10 and save 20% - and it is applied for the buyer automatically, with no code to type.
Open Back Office, then Commerce, then Bulk Discounts, and press New.
Name it, and choose what it applies to.
Either one product, or Library books - which covers every book licence a group leader buys, however many titles. One thing can have one bulk discount; the screen will not let you make a second for something that already has one.
Add a tier for each quantity: how many, and the percent off.
Buying more than a tier asks for still gets that tier. With tiers at 5 and 10, buying 7 gets the 5 tier and buying 12 gets the 10 one. Two tiers cannot share a quantity.
Switch it Live and press Save.
Nothing stacks. The buyer gets whichever is best for them out of the sale price, a coupon and the bulk discount - so a book on an early-bird sale AND carrying a bulk discount gives the better of the two, not both.
The quantity counted is of that one product. Ten copies of one book reaches the 10 tier; ten different books in the same basket does not.
Switching a discount off leaves its tiers alone, so a seasonal one can be parked and brought back rather than rebuilt.