You can checkout using your Debit card in the Credit card option ( it does accept debit cards, it just doesn't show it )
Or use the PayPal option where you can pay with your PayPal account or by using your credit or debit cards without a Paypal account.
All payment are fully secure through Shopify payments or PayPal
Quantity Discounts are automatically applied by a clever little app.
In the Cart area the discounts will be shown correctly for each product, however in the checkout area if you have chosen multiple products it may show the discounts differently (The total will be the same, so you will still pay the same and correct amount)