This app was mentioned in 14 comments, with an average of 1.64 upvotes
The screenshots look good to me For example, look at this app: https://play.google.com/store/apps/details?id=com.samruston.flip It is very simple in design, yet it looks polished and professional. No offense, but yours looks like the initial sketch of the app you want.
This Flip? Seems pretty, but does not seem to have the "tax" calculator.
(by looking at the description and the screnshots, I haven't installed it)
There are many legitimate currency exchange places near the border, but you do always (not just near borders!) need to be careful regarding exchange rates. If you're on Android, I recommend using the app Flip for this; it will cache currency rates (even ones you haven't yet converted to) while online, so you'll be able to get a good idea of what the forex prices are before agreeing to change money. (Note that it's normal to get a percent or two lower than forex changing cash, but if you see disparities of 10% it's probably a scam.)
As for specific warnings:
Laotian Kip is pretty much impossible to exchange outside of Laos, but inside you get very good rates. Make certain to change money before the border, as there will be no money changers who accept it even if you've just walked through the checkpoint.
Cambodian Riel is not used for large transactions, but it is still good to have. Watch out for scams--there is an official-looking building for changing money near the Poipet-Aranyapratet border which gives rates of around 50% of the real value. Again, make sure to change this before leaving the country. It's not quite as hard to exchange as LAK, but close, and the rates you get if you do find a place will be shitty. Major transactions in Cambodia are made using USD, but make sure your notes are crisp and untorn. If you have old looking, torn, or crumpled notes, many places (including money changers and government offices) will not accept them.
Thai Baht will be accepted near the Thai border, but not necessarily throught the entire country. It is usally best to convert it to something more fungible as soon as you find somewhere with good exchange rates.
Thai Baht is pretty easy to change outside the country, but if you're not going to a neighbouring state, you'll usually get better rates if you convert to USD within Thailand first.
Vietnamese Dong are not widely accepted (or are accepted but with a large spread) outside Vietnam, so try to change them before leaving the country.
In general, USD need to be in good condition. When obtaining them, check to make sure they are all uncrumpled and untorn. It is perfectly ok and quite sensible to request a different bill if one of the ones you are receiving looks undesireable. Also make sure to keep them in a proper wallet, so as to not make them wrinkle in your pocket or purse.
Except for Cambodia, using local currency is usually better than using USD, and you may find some places do not accept USD even in countries where many shops do. It is always best to carry some local money in addition to USD, even in Cambodia.
I use flip. Linkme: flip.
edit: https://play.google.com/store/apps/details?id=com.samruston.flip&hl=en
Close, link bot, but no Cigar.
MarionCast, I was talking about https://play.google.com/store/apps/details?id=com.samruston.flip
have you tried Coincalc
Coincalc maybe? It's from the dev of Weather Timeline and Flamingo.
Linkme: Coincalc
Edit: Looks like the bot isn't working atm: https://play.google.com/store/apps/details?id=com.samruston.flip