in the uk at least printer ink is a very competitive market and the suppliers set score by mostly offering free next day delivery, in fact if you do not offer this for anything other than the rubbish end of the market I doubt you would be competitive; suppliers can source from more than one warehouse and so deliveries can be split to avoid delay; in fact amazon are as likely to do this as anyone in my experience

and lets face it the profit margin on the printer makers genuine ink is high... for my brother a set of brother cartridges is around 90 GBP - I would not buy cheap, but I can buy very good compatible for 49 GBP -perfectly ok unless you need to be scrupulous about colour fidelity and long term lightfast