Customer service is for the customer, those that purchase the product a company sells hence the request for the receipt. As margins tighten, companies still want to provide good customer service, they just want to focus it to those who are actually the original purchaser.
They understand if you won the kit new at an event or given as a gift...you just have to explain that when requesting replacement parts.
Those who are purchasing replacement parts, remember that they have to strip an existing complete kit for the parts you need. Sometimes those parts may not be available if there isn't a kit sitting on the shelf or the spares kits have had those parts exhausted already. Then a time lag between receiving your request and the arrival of another restocking of the kit may delay you receiving the parts you've ordered.
Should the kit be OOP then you're just out of luck. You might have to do some creative resourcing.