azizsm:
I do not think the load should be any problem since the motor can provide up to 48Ncm torque and the rolling resistance of the wheels should be on the order of 0.001 to 0.01 as far as I know.
Rolling resistance depends on the vehicle weight, and is seldom negligible. You also
have dynamic loads due to accelerating the mass of the robot which have to be allowed
for.
With a 5kg load you have a lot of mass to accelerate so you need to set the programmed
acceleration to a low value to keep dynamic loads down I think.
Also with a stepper the pull-out torque at standstill is always a lot greater than
the pull-out torque under load, especially if there are vibrations and resonances
to deal with.