My thoughts put her as a busy, slightly disorganised but conscientious person. The fact that there was a payment made without 'encouragement' from you Smee, says to me they had not forgotten you at any time.
Just the priorities of business, perhaps, delaying her in addressing the matter.
Depending on the product area and your financial setup, I would consider her a valuable customer, but make a note of her circumstance and the possibilities of delays.
I would be most certainly sending her a message of thanks for being so conscientious for reimbursement for the extra postage cost.
To have come out of this having only lost a bit of your time and gained a little more experience is pretty positive outcome these days, IMO.