Exodus 19:5 "Now therefore, if ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine:"
It's more than slightly difficult to obey God's voice and keep His covenant. Why? Because we are all sinners. But God has redeemed us with the blood of His son, Jesus. Does that mean we should not strive to keep God's commandment and obey His voice? Certainly not.
I once heard this story told, as an analogy for offering (names are fictional):
Matt Johnson and his wife were in need of a babysitter for their two children. When they found Jessica, she agreed to babysit for them when they needed her. They paid he $15.00 an hour. She washed dishes, did laundry and took great care of both children. She was a wonderful babysitter. However, Matt and his wife soon had another baby girl. For the increased amount of work, Matt talked to Jessica about he pay. After asking her what she thought was reasonable for the addition of watching his baby girl, she replied with, "Mr. Johnson, I'll take whatever you will give me."
Did Jessica expect to get less than what she was already receiving for her babysitting job, just because she told him that she would take whatever he gave her? No, she was going to take whatever raise she would be given.
In this case with offering, just because we are no longer required by the Old Covenant to tithe 10% of all the money we receive, that does not mean that God is looking for us to give Him less. We should be striving to give more.
In the same way, we shouldn't live our lives without any regard to our Savior just because we are saved by grace (Ephesians 2:8-10). No, we are not under the law anymore. Jesus made the atonement for our sins and broke us free. But are we going to live our lives obeying God's voice, or will we give less of ourselves, because it's no longer required?