Before you spend a bunch of money have you tried to confirm the diagnosis? Is there hot coolant present on both heater hoses? Hot on one side, cool(er) on the other? Is the air flow normal?
If they want to replace the heater core there can only be two things wrong. No coolant flow, or the core is plugged with dirt, dog hair, or debris. If the core is plugged have you ever changed the coolant? Mixed it with different brand?
You can disconnect the hoses and run a garden hose backward through the core to see if it is plugged. That is free, and takes a couple minutes.
If it is plugged with hair etc. there is only one way to fix it, pull the core and clean/replace. There is no need to replace the evaporator core, they rarely fail. However if it has debris, it should be cleaned. If your truck does not have a cabin filter they should install one using the Geno's kit, or just order the door and cut the hole.