Paul changes tactic and starts telling the Corinthians how he's been bragging about their generosity to the Macedonians. He's sending some other disciples to pick up the money, and he hopes they won't be disappointed. Of course, this is all freely chosen, it's just that he'd look awfully silly if he boasted about this and didn't deliver. Besides all that, it also helps them to get into heaven, because God loveth a cheerful giver (v. 7).
Paul claims he's shy in person, but bold in print, so he hopes he won't get into any confrontations when he finally puts down his pen and heads for Corinth. His only weapons are his ability to hector people into converting just to shut him up. He also promises to punish the disobedient in the future.
Apparently, there is already a schism as Paul is writing, with each side saying they're the true heirs of Christ. He urges them to see that they're all christians. He says he's not trying to frighten them with his letters, he just feels more comfortable writing, as people have said his speeches suck. He does promise that he's been practising and he'll be better in person. However, they won't go too far, just evangelising.