Here is a model form , we added a sentence in the end mentioning an offer, thanking him all the same and asking for a reply anyway, where he might tell you why he could not help you. The record books you need may be missing, and this piece of information may be helpful too. Print our lists if available for your place etc. It may also be useful to have the names of nearby places. But always ask the priest for permission, some are jealous of their books.
Digital photos is another solution.
There are usually the Liber Baptizatorum, the Liber Mortuorum, the Status Animarum, the Liber Matrimonium. Others may be of no utility, but if the family was prominent you might find also a confirmation document interesting. If he refuses, leave the money in the offer box. The marriage usually took place in the bride's parish. If you are looking for one Antonio or Maria you'll have hundreds of names to check.