Hi,
I have used stripe before for a project for a pizza delivery company and I have fully integrated it with their online ordering system. So I could do the same for your website. As for the design, I often use a framework called bootstrap js which as well as being fully customisable to match your needs, it also enables me to produce websites that are mobile and table friendly. Although you might think that very few people would submit an essay for marking/correcting via their mobile, quite a few might have a look around your website on their mobile before deciding to deal with your company. It is therefore quite important and shouldn't be overlooked.
If you have any questions, please do not hesitate to send me a message.
Kind Regards,
Etienne