I was wondering and I thought it was impossible to purchase the mighty 30 days free-elevator pass using leftover phone credits, but I just recently found out it is possible, and simple.
You simply need to select the PayByCash method. Afterwards, you are given a lot of different payment method, including..Boku. The steps to follow are quite similar to the simple energy purchase using Boku. Only one thing to note: Purchasing using Boku doubles the cost of the transaction. I still find it's worth it, since personally, my leftover mobile credits are lost at the end of the month.
Hope people who also ignored that will enjoy it like I do.