In my business, I send a proposal for an item, the client pays and then I place the order with the vendor.  I do my best to give my clients a full cost outlook for each item, but additional costs such as storage, shipping, insurance, and unexpected labor do occasionally happen.  Here are some costs that you should expect to run into.  Plus, I am telling the world, so they are not really hidden.
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‍Shipping:
Shipping is largely calculated by weight & distance.  If the vendor is unsure of how much the package will weigh until its packed up and ready to ship to you, shipping will usually be billed later.
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‍Storage:
To get that "blown-away-I-can't-believe-how-awesome-my-house-looks-and-you've-even-lit-candles" moment, the install should happen all at once, preferably while you're away so clients can enjoy the impact of what the space looks like finished.  To get the "oh-my-god-I-never-expected-my-home-to-look-this-good-ever" moment, your stuff has to spend some time in storage.  Tucked away until everything is in and ready to be installed for the big "ta-da".  Trust me, it's so worth it.
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‍Insurance:
It's important to have insurance and to check with your insurance company and interior designer as to who is responsible at what time for that Rs. 2,50,000 Coromandel screen you're having shipped from the Far East.  Just sayin'.
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‍Unexpected labor:
Sometimes we as interior designers, can't possibly foresee everything (why is it so hard for me to admit that?).  Perhaps something was hidden behind a large family portrait on the walk-thru, or covered up by wallpaper, maybe that Coromandel screen needs a little repair work but it's not clear how much.  Things come up and you've got to be flexible (anyone who has ever remodeled knows this all too well, right?).
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‍Expected labor:
‍Vendors don't know how much time and effort it'll take until they see what they have to do.  Is that vague?  Maybe restoration work does need to be done on an antique, maybe the silk fabric needs knit-backing, perhaps a lantern needs powder-coating, or rewired.  Many of these things we can get estimates for, but until the work is completed it may not be possible to get an exact cost.
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‍Minimum Fees:
‍Sure, you can order a yard of fabric, but it'll cost you extra with many vendors because of their minimum yardage requirements.  It may seem unfair, but it is time-consuming and not as lucrative to cut one yard as opposed to twenty.  You'll also see minimum fees if purchasing from a wholesaler and not reaching their minimum order requirements.  And unfortunately, I often see vendors providing quotes without this minimum fee, only to have it show up on the invoice when you are ready to purchase.
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‍Rush fees:
If you want to be at the front of the line, you'll have to pony up.  Disney and the airlines do it so this one isn't too surprising.

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