This seems like simple maths to me.  Company report shows that customer acquisition costs are recouped in 18 months ie. customer is profitable in 18 months.  The only things dragging on whole of company bottom line is new customer acquisition and the associated acquisition cost.  Figures are clearly reflecting those acquired customers now shifting to profitabilty, outstripping acquisition costs to the point of profitability (achieved Dec 18). Bottom line, the company is going to be a super profitable company, sooner rather than later as acquisition numbers slow (currently single digit growth).

My only question, as market penetration peaks and 18 months later when revenue  growth slows what will it require for growth?

I'm not sure what sort of market penetration the company has.  Whether they're agnostic into which churches/religions acquire their prodect etc.  The only comment I've seen from the company about future growth strategies is from the CEO in the latest market guidance:

“Given the strength of the underlying business, Pushpay is well positioned to capitalise on opportunities to accelerate growth, including potential acquisitions that add significant value to the current business"

It will be interesting to see the future but I think the current price is well undervalued, likely by more than my current valuation showing an 11% upside.