This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
UNDOCUMENTEDPOSTAddFreeLicense
https://api.steampowered.com/ICheckoutService/AddFreeLicense/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
StoreItemID No
uint32 No
uint32 No
uint32 No
uint32 No
uint32 No
uint32 No
https://api.steampowered.com/ICheckoutService/GetFriendOwnershipForGifting/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
StoreItemID[] No
uint32 No
uint32 No
uint32 No
uint32 No
uint32 No
uint32 No
UNDOCUMENTEDValidateCart
https://api.steampowered.com/ICheckoutService/ValidateCart/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
int64 No
StoreBrowseContext No
string No
int32 No
string No
StoreBrowseItemDataRequest No
bool No
bool No
bool No
bool No
bool No
bool No
bool No
int32 No
bool No
bool No
bool No
bool No
bool No
StoreBrowseItemDataRequest No
bool No
bool No
bool No
CartGiftInfo No
int32 No
CartGiftMessage No
string No
string No
string No
string No
int32 No
fixed64 No
bool No