Royal Mails own lifting and handling training clearly states this,
The acronym T.I.L.E is used to determine this.
T - Task, Can you as an individual carry out this task safely?
I - Individual, Is it in your own ability to carry it out safely? (Carry out your own risk assessment)
L - Load, Can you safely lift, carry comfortably without strain the item? (Consider Shape, Size & Weight)
E - Environment, Consider the working environment I.e steps, ice, slopes, debris, leaves etc.... is it safe for you on your own to go down steps with said item if you struggle to lift or hold it?
Hope this helps you now or in the future, after all your safety is top of Royal Mails priority