Advise from retired florist, also mother of bride-groom 5 times. Most wedding party paticipants do pay for their own attire. Special sashing at last minute is bride being bridezilla. Sorry to be so blunt. I would suggest your daughter tell her she can not afford it nor can you. See if she offers to buy it. A lot of brides get very demanding the closer it gets to their wedding. I suspect dress store is charging lots of postage for last minute add on and shipping.
Good luck