Online shopping

I have placed an online order via the above platform. My first order was rejected hence me resubmitting my order. When I checked my basket it showed 1 item and have requested 3 of that specific item. I have placed a second order as I have forgotten a few items. The amounts can be quite confusing as they have a reserved amount as well as an actual amount being debited to your account. 4 amounts were deducted from my account but 1 was reversed once the actual amount was received. When I contacted the call centre, I was told how the process work. The consultant informed me that if the reserved amount is not reversed the same day it can take up to 10 business days and need to inform the bank to reverse the amount and they will provide me with release letter if it is required. What if I did not have extra funds in my account for the reserved amount and why must I wait that long for the amount to be reversed and why must I contact my bank to reject the amount. It's the first time I have experienced this kind of online shopping and will the last I will make of that platform again. Someone needs to explain why I was charged 3 amounts for 2 orders placed. It's been 5 days and the funds are still not back into my account. Unacceptable!