Hello Tyler,
From your writing style it appears that, you already know the minimum basics of SSL/TLS an HTTPS.
Now coming to your questions, all SSL certificate provides a kind of 'insurance'. A good list for comparison of SSL certificates on Wikipedia can be helpful. This kind insurance is called indemnity. This list will partially provide you some idea why the pricing varies. Most modern browsers including mobile browsers on mid to high end devices will support SSL if purchased from a good brand.
Purchasing using phones are separate service, you can use any good Payment Gateway.
Unless you have any special reason, practically you can use Paypal Merchant account without really having a SSL certificate.
You can talk about these on this Google Plus Group - https://plus.google.com/communities/111285810442781953487 (its named Building a Company and I am not the owner of the group but a member), You will get better advice from a big business owner user base.