How translate the specific business word "économie d'échelle". It is the system which with more you buy on big quantity, less you pay per unit.
I think we'd say "volume discount", i.e, a discount from the regular price because of a large-volume purchase.
I do not agree it's not necessarilly about production and can indeed be used for purchases.
"economy of scale" describe the case where cost can be split into a fixed part and an proportional one, so increasing the number reduce the fixed part per unit:
placing an order: $10
price per unit: 1$
an order of 10 units cost $20 ($10 for the order and then $10 for the 10 units) therefore the cost of of one unit is $2 (20/10)
an order of 100 units cost $110 ($10 for the order and then $100 for the 100 units) therefore the cost of of one unit is $1.1 (110/100)
economy of scale allows you to save $0.9 per unit
Is this anyhow clear?