Paper checks are cheap for recipients, depending on volume, because the processing fees are really low. An individual simply needs to scan the check with their phone. Processing large volumes of checks (e.g. thousands per month) as a merchant can be outsourced to a lockbox facility if needed. Payers can write and mail a single check in a couple minutes or use a bill pay service that will do it all for you with a few clicks.

Banks are trying to replace checks. Zelle, which sits atop ACH, is an effort to simplify P2P transfers. Real time payments (RTP) are on the way, and will have some invoicing functionality (e.g. line items). The challenge is to get enough banks, covering enough of the population, to make the system feasible. Zelle or RTP don't succeed if I have to fall back to using a check or cash because my friend's small bank/credit union does not support the service. Also, this all requires an investment of time and money to upgrade backend systems. That investment is easier for some institutions than others.