They offer a one time purchase for "premium" (I don't remember the price because I purchased ages ago using play store credit I got from surveys mainly to support the devs). Here's what the documentation says about it:

Free edition The free edition only has a single limitation, a maximum of 30 running blocks, that could be 15 running flows with two blocks, or a single running flow with 30 blocks. Unused/unconnected blocks within a flow are also counted. There’s no limit on inactive flows stored on the device. All features can be used with or without Premium purchase (if supported by device).