i think 4-5kg still sounds okay. but i think not anything more than 4-5kg, anything more than that will consider HEAVY IMO.
and the posture is very impt too. dont ust bend down and ift the stuff up, squat, pick, slowly stand up. is it possible to seperate the 4-5kg? as in, seperate to 2-3x instead of carrying them all at once?