I like to be fair on shipping and will only ever charge the actual postage cost (or less).
For mint sheets in my store, I like to offer a flat rate of $4 for any number of sheets you purchase with one order. Unfortunately, the postage calculator on Hipstamp doesn't like that and always tends to over-calculate the postage. If I catch it before you pay, I can override the system though and update your invoice to reflect the $3 flat rate postage for sheets.
Similarly, single stamps, covers and plateblocks can often be combined for much cheaper than what the postage calculator reports. So, please let me update your invoice with combined shipping before you pay for multiple items.