I have some experience using POS card readers.

AFAIK, POS credit card transactions are only supported from 9 to 12 using Mercury Payment System, and (by the time I'm writing) it's only supported using magnetic readers, no EMV credit card (chip) readers are supported also only USA & Canada are available.

You may use POSBOX (now IoT Box) or connect the magnetic reader to your POS machine directly since most OS support the Dynamag readers which are currently used.

If you want to use and EMV reader then you may setup and Credit Card account, enable it on the POS, but don't install the Mercury Payment app, so you can manually charge the amount on the EMV reader then manually validate the purchase.

• Does that leaves room for error?, yes.

• Is there some other credit card system working for Odoo?, not that I'm aware of.
  

That may change soon for version 13, which may be available world wide and also part of the Enterprise features¹.

¹ https://github.com/odoo/odoo/issues/28179#issuecomment-453936928

Cheers!

connecting and running  payment terminals on POSBox in different countries is heavily underdocumented.

this is what i know:

to connect a payment terminal you need three things:
1.
hardware: POSBox  and payment terminal.
if POSBox has no free USBs port for the payment terminal you need serial connector cable. (this problem is not 100% clear).

2.
additional module on server:

 "On the main Odoo server, you should install the module pos_payment_terminal."

https://www.odoo.com/apps/modules/10.0/pos_payment_terminal/

this module talks to the POSBox that it shall communicate with payment terminal.

3.
additional module on POSBox:

On the POSBox you may install a module with the right protocol for your payment terminal IN YOUR COUNTRY!


"On the POSbox, you should install the module hw_x depending on the protocol implemented in your device."

for France, the Telium protocol seems to be running. you can use module hw_telium_payment_terminal :
https://www.odoo.com/apps/modules/8.0/hw_telium_payment_terminal/

(it is not sure whether that works on 9.0 or 10.0)

if you have other than Telium protocol (e.g. ZVT, OSI in germany) then you must ask for a module that supports that.

i have not found any other module than Telium.

--
hint: call the biggest odoo gold partner in your country and ask them directly (and provide answer here).