Use different receiving addresses for different customers as I wrote
here and
guide from nc50lc to get QR code with Electrum wallet on Desktop and Mobile.
If you have yet felt clear about that, let's see the screenshots.
Above is how the wallet looks like after I assign three receiving addresses for 3 customers from Customer1 to Customer3 and how you can get QR code.
If you set up each address with a specific amount for one customer, tap on URI to get the QR code for that customer. If you get QR code by tapping on Address, that user will have to type amount by himself.
Bitcoin URI Protocol Scheme
One note for OP.
If you want to be safe, after a while of using your first wallet to receive
BTC from customers, you can consolidate their inputs and send them all to a new wallet that you won't expose it to the Internet and customers too often. Use that second wallet like your company treasury wallet (if better, use a multi-signature wallet for it).
It's similar to how centralized exchanges move customer deposits from their Hot wallets to their Cold wallets.