It's a shame you're in a tizzy over $29.
Many have said that only you know what/how your husband will feel if you tell him about your lie and that is true. We don't know what else is happening in your life that $29 can make you worried about his reaction.
You can say you made a mistake or mis-spoke and that the actual cost was $XXX and just wanted to clarify the amount so he won't be surprised when he sees the receipt or bill on it ( because it was a mistake to lie (IMHO) ). If he is going to have a fit over $29 and make your life miserable for several days, you KNEW this when you lied and also knew you were taking that chance.
Oh well, nothing we say or suggest can sway your decision. Read your tag line in your signature and go from there... good luck and best wishes.