Pre-purchase check if custom instructions required
Customers set aside time for system installs. If, partway through the install, they are suddenly told they have to wait two business days for instructions, they have to set aside more time to do the install.
I understand why you don't provide instructions to customers that have not purchased the system. However, you could at least indicate before purchase if custom instructions are required.
Allow customers to select their configuration of existing thermostat/boiler and make a request if instructions will be instantly available or not. That way, a customer can properly sync scheduling of both purchase and installation of the system with the receipt of the instructions.
This is especially important if, for instance, a customer wants to install on a Saturday. On that Saturday, they'll find out it will take two business days for instructions, which means at best another four days (Saturday/Sunday/Monday/Tuesday) before installation is possible.