I remember reading a study once that concluded that buying from a farmers market is only emissions negative if the farmer lives within a few miles of the market. Ship transport is mind bogglingly efficient.
So, if a plastic straw costs two cents, then at most about 1/200th a gallon of oil/gas was used to both make the plastic and ship it to wherever you are.
It is very easy to look at the visual volume of straws or bags and feel like it is a lot of plastic - but in reality it is the equivalent amount of oil usage as driving a few more blocks.