Virtuemart customer choses price to pay (for flowershop)

Hi, i need an input-field where the customer can enter the price he wishes to pay as alternative to the given prices. Pls have a look here what i mean (site in french; => it's a flower store): [login to view URL]|utmccn=(organic)|utmcmd=organic|utmctr=fleurop.ch&__utmv=-&__utmk=163552956&uid=105745794&lang=FR There are 4 price variants. 3 of them are no problem with virtuemart: "minimum", "le parfait milieu" and "de luxe". what i need is a modification that allows the 4th price field ("au choix"): the customer can enter a price by his choice in an input field, and this price will be taken by vm. Would be best to have a custom field that allows this as not every product needs this text-input for a price by choice. the only restriction is a minimum-price. Looking forward for replies. Best regards, Daniel