“Plus-plus” is a catering term indicating that the price mentioned for each person does NOT include service charges and/or gratuity and tax.
Plus-plus can be slightly misleading to the untrained engaged couple and is usually written like this: $55 ++. In this instance, the cost per person is $55 PLUS service charges and PLUS local taxes. In essence, the $55 is simply the base price.
Service charges vary, but tend to run in the region of 20% (ouch!). The rate of tax varies by each US state.