Thanks for explanation.
How do you plan to make money then?
It seems that the business model you are building is not really sustainable because any scenario (blynk or any other platform, or self-development) involves monetary investment and recurring expenses which are not then covered by your customers.
Unfortunately, Blynk can’t provide you with a way to run your business at 0 monthly expenses or with a one-time payment.
Maybe you can reconsider the business model and either charge your clients, or include platform costs into the hardware costs, or provide additional premium services.