To increase the character limit on the price adjustment field open the "productvariants.ascx" page and scroll down to line 341 and change the value for the "MaxLength" property to 7 or 8 as follows:
<asp:TextBox ID="txtPriceAdjustment" runat="server" MaxLength="7" Width="100" />
We will include this change in the next release of the cart.
Thanks,