First mate is the highest rank you can buy in-game, once you have it, that’s it you are firstmate forever! The only next logical rank you could obtain is quartermaster, but its hell-a hard to get! So you are the highest paid rank you can be, if you wanted to go higher you could get a donation rank that goes over the top of Firstmate 🙂

Not sure if this is a bug, but I’m pretty sure you’re not supposed to have those rankup rewards if you purchased a rank. I believe it’s only if you actually worked to get the rank.