Update purchase.go

[casalsgh]

Changes made are mostly related to code style and naming conventions:

Struct field names are now in camel case to follow Go's naming convention.
The comment for the OneTimePricing struct has been updated to clarify that it represents the pricing for a one-time purchase.
The NewPurchase function has been added as a constructor for creating a new Purchase event with the specified values. This allows for easier creation and initialization of Purchase instances.

These improvements enhance code readability and maintainability while adhering to Go's best practices and conventions.