Can Webtix charge just ONE service charge if patrons buy more than one show at a time?

The quick answer is no. Each order (or sale transaction) has its own unique sale number. The per order service charge applies to each unique sales transaction.  Webtix can perform multi-sale purchases at one checkout, but the $4.50 service charge is applied to each unique transaction.
We recommend that you put the service charge on a ‘per ticket’ basis instead of per order. That would make these online transactions more affordable for your patrons.