I spent just under $30 all told, and at that price point, for Vietnamese Dong, I believe that about 37% of what I spent (around $10) went toward the clerk's time, the lease, and other costs of doing business in a physical storefront, maintaining dozens of currencies. In other words, I paid about $10 for the convenience of obtaining currency usable on the other side of the world on my lunch break, at a moment's notice.
As a former bank teller, I can attest that currency exchange clerks are doing real work (as regards cash handling, it's more complex work than they would be doing at a bank). Furthermore, it takes some real effort to put together a system for this sort of thing. Additionally, I am not certain how much of the commissions must be set aside to pay currency exchange taxes.
I think what is most helpful is to go into this sort of thing informed of one's alternatives. For me, today, I got a value that I'm satisfied with. If I regularly needed the same currency, I would look for a way to obtain it that had a smaller effective commission (a smaller buy-sell spread, a smaller service charge, or both). In fact, today may be the first day that I actually used Travelex.
(I recognize that Travelex does not mention its buy-sell spread and certainly does not refer to it as a commission: in fact, its FAQ page oversimplistically disavows the existence of commissions in its business model; likewise, it has a flat-rate "service charge" and simultaneously claims to have no fees....)
In some countries, especially on return visits rather than first-time visits, I would feel comfortable merely withdrawing cash at local ATMs, for which I would hope to pay less than 1% to VISA or MasterCard, and maybe a small fee to my bank as well.
But I am grateful that Travelex exists and know that, for the ready foreign currency it offers, it offers a better value than many banks, which may have flat fee around $20 in additional to a large percent-based commission. Another option to consider may be AAA (Triple A), if you are a member. I am not certain at the moment which would have been cheaper for the transaction I made today.