RS Components double-debits bank accounts.

I recently used my company debit card to order components online. RS immediately reserved the purchase amount (incl Vat) of R8157-71. This amount was move from my available funds, to the 'uncleared funds' part of my bank account. RS has now dispatched a portion of the order, which has not yet been received. They have not drawn that portion's funds from the uncleared, reserved monies. Instead, they have taken another R4,128.55 directly from my available funds. When the second part of my order is dispatched, they will again take an amount of R4,029.16 from my available funds. Effectively, they will have withdrawn the amount of R16,315.42 which is double the purchase value. This happened once before and it too many months and phone calls, to get my money back. How can this be legal? How can the bank permit this?