SRL Diagnostics — Invoice copy

We are getting tested the blood and health check up of our entire family last 2 years. The report looks fine. But problem with invoice copy. It has been noticed every time that payment invoice copy has not been issued to us. Every time that requires push from customer side to get the invoice copy. If customer forgets, that becomes the benefit of SRL.
I am not understanding why SRL is so reluctant in issuing Payment invoice. Why customer needs to remind SRL for invoice?

Invoice should be issued immediately after payment. If not issued .. that means something else