Need to be able to sort products in cart by manufacturer, then subtotal each manufacturer dollar total and compare the total to the assigned minimum order total amount for each manufacturer.
If the amount is equal to or greater than then allow that portion to check out, but if the subtotal is less than the assigned minimum, then have a warning display and alert the shopper to select more items from that manufacturer to raise the total above the minimum.
This may require altering several PHP and database files and should only be considered for moderately experienced programmers.