Hi,
Unfortunately, I’m not sure there’s any workaround at the moment, outside of making modifications to the engine.
For projects that intended to implement purchasing purely in the OSS, the intent for ISteamPurchasingServerLink was to allow this kind of project to support Steam with minimal modifications. If Steam is the only platform you intend to support (or you already have your own purchasing flow), another option could be to handle purchasing in your own project code instead of using this interface.
Thanks,
Alex