"an afterlife makes virtue mercenary"

Or, in other words, if religions (not just Christianity) promise rewards or punishments in the afterlife for deeds done now, doesn't that reduce good deeds to be "services rendered" in exchange for personal gain?

Sure, but only if a happier afterlife is the motivation for a good deed. In Christianity, no good deeds are sufficient for admission into heaven. It's true - Holy Harry, the upstanding community figure, can't cut it any more than anyone else. As Jesus told his listeners, they had to be more morally correct than their society's experts on moral correctness, the Pharisees and teachers of the law, in order to be part of the kingdom of heaven. The only way to make it in, is for the Christ to cover your unrighteousness with His perfection - known as "justification". Given that this is a central tenet of Christianity, no Christian (who knows what he or she is doing) should be consumed with concern for earning what is essentially a gift.

Does this mean Christians no longer need to care about what God thinks? Read some of Paul's epistles on the subject to convince yourself that forgiveness is not a "green light" for doing wrong (confession is vital, but the aim is to have progressively less to confess!). Again, this is an annoyingly common mistake. Sin is separation from God, meaning no fellowship with God. Hell is the soul's eternal separation from God. To put it frankly, the removal of sin is not just for You, that is, for your benefit. The sin is removed so people and the Holy God can come together. Someone who wants forgiveness for sin without pursuing a relationship with God is missing the point entirely! Not to mention that asking forgiveness is rather deceptive if you plan to go on doing what you asked forgiveness for. If you honestly, humbly desire to be a follower of God, you won't continue to do wrong, because that is the opposite of following God. Your moral compass has been corrected; why are you still going in the wrong direction, pray tell?

To repeat, hell is the soul's eternal separation from God. Heaven is then the soul's communion with God. The fact that God is good and the Source of all good things can make it seem like desiring communion with God (i.e., Heaven) is the same as desiring good things for you. Remember that according to the challenging book of Job, Job's trials are precisely to establish that Job honors God because Job's goal is to honor God, and not because Job wants God's blessings. So there is a difference. Someone who's making mental calculations like "if I do good deed C which costs me Q, I'll receive reward Z in the afterlife" has the wrong attitude. Just as people who correctly observe the root selfishness of that approach are not fooled, God is even less fooled.

A useful analogy is any act of sharing. People can contend for any resource, taking the whole by force, or they can agree to take a reasonable fraction and live peaceably, which is a better solution for everyone. Does the fact that sharing is a better solution for resource allocation imply that sharing is motivated by selfishness, out of the economic choice to certainly gain a little instead of risking injury and likely receiving nothing?

Coming to the point, the Good should be desired passionately, but such desire is not greed! Greed (or selfishness) is desire without limit. There can be no paradise for all if one individual refuses to appropriately curtail his own lusts. The believer should not overindulge, being mastered by addictions. But the believer should not overindulge in self-denial, believing that the highest ambition is to never be happy. Other belief systems may mention a Middle Way; Christianity's path is not so different in that respect.

Desiring to cultivate and experience the Good of harmonious moderation, and the ultimate of which is an afterlife with the Good God, is nothing to be ashamed of. Desiring to notch up points in the here-and-now in order to gain more for oneself in the hereafter, that is something to be ashamed of!