Board Thread:Roblox help and discussion/@comment-39692739-20190604010713/@comment-35998649-20190604012331

Are you talking about the "Supercharge your Purchase" option when buying BC?

I just checked and the only options are
 * 1,000 Robux
 * 2,750 Robux
 * 6,000 Robux

It might just be an old offer that Roblox removed at some point. I'm not too sure.

Edit: I see what you are referring to. This link shows 10K Robux plus the 5K Robux offer. I think this means that first you have to join BC, and then purchase the 10K Robux option. Once you do that, you get the 10K Robux PLUS an extra 5K. (A grand total of 15K Robux)