Shipping options seem extremely limited

Just purchased MarketPress (not Lite) and am reasonably impressed with it so far, BUT I’m very surprised by the apparently limited support for a reasonable range of shipping options. In particular, I’d like to have two shipping choices — standard or upgraded, and I’d like to have a simple formula or table that computes shipping cost based on the number of items orders – e.g., for upgraded shipping, first item shipping cost is $6, add $3 for each additional item in the same order. Instead, all I see is “flat” shipping or shipping computed from cost alone.

Am I missing something? Does basic functionality like what I described require an added plug-in or other mod?

Thanks

Grant