The existence of refunds can be logically explained: The agent has to get money from you before they take your parcel to the post office.
Even if they think they know how much it will be, it makes sense for them to ask for more money and refund you later. The alternative would be to take your parcel back to the warehouse and ask you for more money, in case it turns out to be more expensive. That would be annoying and slow.
Keep in mind, that there are many reasons the agents are unable to tell you precisely how much it will cost:
- they haven't even physically put together your parcel (this is the biggest factor and rehearsal shipping helps with this).
- fluctuating prices
- fluctuating currency conversion rates
This makes a lot of sense and you'd probably do the same if a friend asked you to ship a parcel for them.
The refund will be credited to your account wallet.