They clog up windshields, cause temper tantrums and wind up costing Island residents a small fortune each year, but who knew those little purple parking tickets — the dreaded and despised staple of Vineyard summers — were such an important money-maker for the county? Plymouth County, that is.

Anyone who has ever failed to pay a parking ticket on time has likely received, at least until recently, an overdue violation notice from the Plymouth County treasurer’s office. For those who noticed, it may have seemed like an oddity, since the initial parking violation occurred in Dukes and not Plymouth County, which lies two counties away.

The reason was a matter of simple economics: Dukes County saved money by outsourcing the processing and collections on parking tickets to Plymouth County. Several years ago the Dukes County commission signed a contract with the Plymouth County treasurer’s office to process all the parking tickets issued on the Island.

Under the agreement, Dukes County paid a flat fee of $1.50 for every parking ticket processed by Plymouth County, annually generating between $12,000 and $20,000, depending on the number of tickets written. Plymouth processes parking fees and fines for more than 30 Massachusetts communities, and the Vineyard was its biggest customer.

Until recently. Late last year Dukes County announced it was ending the arrangement with Plymouth and would begin processing tickets in-house.

On the surface it again came down to economics, due largely to the state takeover of the county sheriff’s department. Because of the takeover the Vineyard lacked funding for its parking clerk staff; the decision was made to make up the shortfall by reclaiming the fee generated for processing tickets.

County parking clerk Carol Grant said this week said the change would save about $18,000 from parking tickets processed last year alone. She said every bit helps right now, with revenue for the county down in several areas, including parking tickets.

During fiscal year 2009 just over 13,000 tickets were written, generating $292,000, including tickets paid from previous years. In the first four months of fiscal year 2010 (June through September), when most tickets are written, just over 9,609 tickets were written generating $168,000.

All six towns increased parking violation fines before last summer.

But while belt-tightening is the main cause for making the shift, Ms. Grant acknowledged there were other problems as well, including complaints about Plymouth County’s poor performance in ticket processing.

The problems apparently can be traced back to two years ago, when Plymouth County transferred operational authority for its parking department to the county treasurer’s office amid concerns about staffing due to budget pressures.

But more problems arose last summer when budget cuts forced the treasurer’s office to reduce its own staff. Parking ticket processing fell behind, and angry complaints, many from the Island, began to pour in. On Feb. 5 Ms. Grant sent a letter to the Plymouth County treasurer announcing that Dukes County was ending the arrangement.

“The service has not improved and our concerns continue. As of this date we have not received payments for November or December 2009,” Ms. Grant wrote.

This week she was diplomatic about the dustup.

“Let’s just say timing was everything. We were looking for a new and consistent source of revenue. There were a variety of reasons we ended the relationship, but the main motivation was financial,” she said.

County manager Russell Smith said he was hearing plenty of complaints. “People were waiting months for the tickets to be processed, so the level of service did play a role in our decision. But this also was an opportunity to save money . . . that was huge for us,” he said.

Newspaper reports early this week showed that Plymouth County leaders were unhappy about losing their lucrative contract with the Vineyard, and exchanged heated words about it at a meeting.

On the Island the transition has been much less volatile. In recent months Ms. Grant has visited selectmen in the six towns asking them to approve the change, and in all cases the votes were unanimous with little discussion.

“[Plymouth County] has their problems, and we have our own issues,” Mr. Smith concluded this week. “We think this will be a better arrangement for everyone.”