Experienced the same thing a while back. The seller assumed that by asking the shipping cost I had agreed to a price when I was actually trying to find out shipping so there were no surprises after agreeing a purchase price.
I would think that if you asked for the shipping price before saying I will buy the item then you are trying to work out if the total cost is within your budget.
If you negotiate $5 off the price then ask for shipping often the seller just adds the $5 back onto the shipping cost. That has happened to me much to often.
Ebay even allows you to ask about shipping before bidding or buying.
In my case the seller seemed to assume I was asking for a discounted shipping rate
Eg item $180 shipping quote $32 total $212
I offered $200 total
The seller went off a little about me trying to rip him off on postage cost. I tried to explain that my offer was $168 + $32 postage but he didn’t see it that way