Alternative to immediate purchase

• Payment plan
• Purchase payment plan
• Certain payment plan
• Department-store plan, perhaps
• Kind of plan
• Purchasing plan
• Reserve for Christmas, perhaps
• Purchasing option

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1961, as a system of payments for merchandise, from lay (v.) + away . Earlier in the same sense was Australian lay-by (1930).

Wiktionary Word definitions in Wiktionary
n. 1 Things which have had a deposit placed on them and which the owner (generally a store) has agreed to hold for the customer and accept full payment at a later time. 2 The system of accepting a deposit and holding for a customer to complete purchase. ...