while shopping on Sundays is not aloud, i’ve been told by a very good priest i know, that online shopping is not the same, for one because of what you mentioned, that it would not be processed that day, and also because it is in no way laborious on your part,
and for everyone else, yes shopping on Sunday, even for fun,
is a mortal sin, the church has always taught this, and you don’t have to go very far to verify its truth,
While Sunday is a day of rest, it is not only that, it is a day devoted to the holier things, the things of God, and even bodily exertion is aloud on Sunday when it is to help those in need, so it’s really not about what’s “fun” or “restful”, it’s about devoting your time to God, it’s Gods day, and while it’s fine to rest and recreate, that should not be the better part of your day, Sunday is not an excuse to be lazy or to waste your time, there is still alot of work to be done, but it’s just on a more spiritual level.
and of course Sunday is also good spent visiting family and being with your family, but the spiritual should be incorporated into that as well, such as saying the family rosary with your extended family if possible, or taking part in other religious devotions…
but for some really good advice and clear guidelines for keeping Sunday holy, listen to this short but very excellent sermon, and check out the rest to -
audiosancto.org/sermon/20070729-Keeping-Sunday-Holy.html
alright, hope this helps, take care.