Hi lilypad, looking at your products list, your thumbnail dimensions are 250 x 250, so are not being 'scaled up'.
They look ok, but you might get sharper images by setting "Image type for generated thumbnails" to PNG in general settings > appearance (although image file sizes may increase slightly).
Otherwise, you'll just have to re-upload / re-generate the blurry ones.
See
here also.
Also, in the template settings set Products > "Product thumbnail appear effect" to Disabled and see if that makes any difference.
Meanwhile, your checkout issue has been sorted.