"the tribute of the offering of your hand": We are hereby taught that one brings his obligatory offering from chullin (non-consecrated monies).
And whence is it derived that if he wishes, he may include some of his tithe monies? From "as the L-rd your G-d will bless you."
(Devarim 16:11) "And you shall rejoice before the L-rd your G-d": It is written here "rejoice," and elsewhere (Ibid. 27:7) "rejoice." Just as there, the rejoicing is with peace-offerings, here, too, it is with peace-offerings.