Tribute ships

The tribute ship isn't exactly bringing explicit joy, but its very existence and timely and regular arrival contribute to prosperity of the Imperial mainland. 
When the office of the Count of the Overseas was first established, the tribute ships were conceived as a means of circumventing the difficulty of collecting taxes due to the crown as a vassal's contribution directly into the imperial treasury. The taxes would be tallied by the imperial Office of the Purse, and information would be delivered by His Majesty's agent aboard a small flotilla. The flotilla then would be loaded with the amount that the Count of the Overseas was due, three parts in kind and two parts in precious metals and coins, and afterwards would carry the overseas dues to the mainland. 
Thanks to the relatively close position of the overseas possessions and the general ease of navigation between them and the mainland, the amount of disasters that plagued the fleet wasn't significant, especially since the Empire didn't lack for armed escorts to protect the taxation route. 
Thus, the imperial treasury was all but guaranteed an influx of money twice a year - from the overseas produce, the land taxation, the house taxation, the customs, and other revenue sources. Normally tax was levied as a percentage of the Count's revenue plus a flat sum, so its amount and composition could be highly variable. 
But invariably the arrival of tribute ships brought great joy to the people of the mainland, especially to those in the Thirteen Cities that comprised the most prized direct holding of the emperor. It eventually became a custom to hold market days when the tribute ships arrived - the influx of overseas goods, especially the rarer ones, inevitably drew in customers and netted both the imperial treasury and the resellers that had the privilege to buy directly a good profit, and occasionally a donative was generously distributed to keep the population happy. 
Needless to say, the tribute ship arrival was considered a rather happy occasion. With the loss of the bulk of overseas possessions during the reign of Theodahad in the early XIII century the amount of goods and money provided by this usual route decreased drastically, and the market days lost a great deal of their former lustre, but when the ships do arrive, carrying with them exotic goods and extra revenue for the treasury, they are still inevitably received with great jubilation.