Well, i seen such proposal in survey from this post - viewtopic.php?f=9&t=987, but it was asked for long time ago and still no progress.
So, i have kind of stupid idea how it can be implemented in win-win to all way.
As far as i understand, main problem with increasing is that it considered as another payment transaction and subject for payment system fees. If user pay 1$ with 0.12-0.15$ and after pay 0.5 with almost same fees its really not very profitable.
This may be acceptable with Humble Bundle average payments and bonuses, but on IGS BTA is generally 1.5-2.0.
Then, i see pretty easy way to workaround this - implement store credit\balance on IGS. Let user, say, deposit some amount of money directly to his account, with fixed sum or limited minimum or as much as he wants (its details) and let user later to use this deposited money to increase his payment. In this case it will be internal operation so no additional fees.
Not sure about legal matters, but store credit and balance is pretty usual thing, for example Namecheap have such feature.
Only interesting thing in this case is should be BTA increase amount be fixed for user according to moment of purchase (like in Humble Bundle) or fluctuate like on site.
Anyway, i think that this feature may be will not be used as onetime\rarely purchasing customers, but as usual customer i dont see a problem into depositing several $ for be able to increase payment later.