Tips:
1) Don't call anything a donation if you're not a registered 501c tax exempt charity. (You won't be.) Instead, let people buy a supporter button on your home page or a "registered" version of the software with whatever trivial difference you want. (e.g. displays "registered" or "Thank you!" in the about screen.) This is a preventative inoculation against lots of trouble down the line with your credit card processor.
2) If you don't have a specific reason for forming the LLC, skip it and take money as an individual. Many devs have an inchoate fear that this is somehow illegal. That is the opposite of the truth: sole proprietorship is overwhelmingly the most common form of business operation in the US.
3) Do not encourage people to pick their donation amount. Instead, pre-pick three, which you can assign three levels of status to (bronze, silver, gold, whatever -- you and I know it is meaningless, but the customer won't perceive it that way) and price them at modest, generous, and high. e.g. If you are thinking $5 right now, I would suggest $10, $20, $30 or $10, $25, $50. You'll get most of your transactions at silver and make most of your money on gold.
I live in Uruguay, $0.05 is a very common amount ("un peso").
There are basically two ways to do five cent transactions. The first way is to buy a virtual currency for a non-micropayment amount (e.g. $5 or $25) and then subdivide that virtual currency into infinitely divisible bits which you can spend seemlessly within the application. This is the successful, Zynga-esque way to do micropayments, and it sits nicely atop the legacy payment infrastructure, which is entirely suited to doing $5 or $25 transactions.
The other way, where you attempt to make an actual transaction for $0.05, is an operations nightmare. The legacy payment system is fundamentally not equipped to handle it. The conversion rates to purchases will be terrible, because of the "penny gap" (asking people to spend money, and in particular asking them to make a new relationship to spend money, always causes a huge dropoff) and the conversion rate will not be meaningfully higher than if you had charged $5. Customers hate the experience of authentication for $0.05 transactions even more than they hate actually paying money, and common schemes for doing the auth once and then bundling transactions across merchants like e.g. Flattr are basically a non-starter for most users.
(I'm largely speaking about making services for the global rich here. It is possible that the situation in Uruguay is different, although I'd still suggest not getting into this field as a student.)
For example, Paypal has this Micro Payment option: https://micropayments.paypal-labs.com/?bn_r=m
$0.05 cents + 5% per transaction.
Its just not feasible. I can't remember, but Facebook is not helpful when dealing with low payments for game items.
I believe MercadoPago is not in Uruguay. Seems weird.
I'm researching MercadoPago for a personal niche project of mine in Argentina (where I live, hola!) and I found that the minimum amount is ARS $10, which is around USD $2.30 or $2.40.
Sounds good :) and the fees are very reasonable (3.5%)
BuySimple https://www.buysimple.com/ is the one I liked the most (and the one most similar to my own service)
There are a lot of SMS micropayments services with a very high per-transaction cost:
http://www.daopay.com/?language=es
A trade website:
Other stuff I haven't tried:
https://checkout.google.com/inapppayments/
There are dozens of others
