Players should be able to choose exactly how much RP they'd like to purchase
If you've ever purchased RP, you know why this would be good for the players. You want a 975 RP skin, but you have to get a 1380 RP pack at 10 USD.
Obviously this method is beneficial to Riot because players will buy RP they probably won't even use entirely, and having leftovers will also make them more likely to buy a little more to get something else (in a vicious cycle, because they'll probably have extra again).
Even knowing this proposition would be a loss for Riot, I'm going to propose it anyway; Let us buy RP in whatever amount we want. Instead of giving bonus RP for a certain amount spent, reduce the cost directly (instead of +80 RP for a 10 dollar purchase, reduce the cost by X amount for choosing 1300+ RP).