Instructions for completing registration can be found in the link below. , County will pay Contractor for services rendered under the contract as specified in the attached Payment Schedule.Complete registration here to begin using Contra Costa County Purchasing. Vendors, please read this disclaimer prior to completing registration. This form is intended for use with the. This Addendum is intended for use in the unincorporated portions of Contra Costa County. Answer-Contract. POS-010, Proof of Service of Summons. POS-015, Notice and Acknowledgment of Receipt-Civil. The costs to purchase service credit fall into two categories, depending on the type of eligible credit. Purchases can be made with post-tax dollars.