I'm not so sure about that one -- unless you spend all day shopping or something.
The fact is that a web server doesn't need/get to take Sunday off to worship God. So if you place an order, you're staying at home, selecting a product, typing a few numbers, and you're done. The whole thing might only take 2 minutes. That is certainly not servile work.
If you're buying CATHOLIC items, then it's even a good thing. Some people are busy, and they won't otherwise have time to browse for statues, books, music, etc. It can be very relaxing for some people to surf the Net on Sunday. Some people actually enjoy browsing, especially for objects they are interested in.
This presumes that you also go to Mass, avoid servile work, etc.
Matthew
Matthew, I didn't mean that web servers had to keep holy Sundays and Holy Days of Obligation (I'm sorry but that's how I understood what you had inferred I meant). Web servers, however, do not direct us to fail to keep Sunday's and Holy Days of Obligation holy, we do. MOST forms of public commerce are forbidden on these days. (The web would be considered public, even while people often use it in the privacy of their own home.) Direct advertisements via emails is planned by the companies, not the ad agencies they hire or their long-distance data communication providers. When a company has an email campaign targeting it's prospective consumers and asking them to make a purchase of a service (which is not an essential) by or on a Sunday or a Holy Day, this is not morally correct. Here's an example: a book for spiritual reading is acceptable, since it is to orient oneself both exteriorly AND interiorly to God, however, purchasing "Catholic clothing", such as a tshirt with Catholic insignia, or (for the women) a long skirt, this is not the same thing as the last is not keeping the interior directed to God. The last example is superfluous, although we need clothes, wearing them doesn't orient ourselves to Christ, it's just a necessity. If all our clothes were burned in a fire, we'd still have Monday to purchase new ones, or if not, we would require dispensation.
While it's not servile work, but it's profaning Sunday and Holy Days.
We aren't supposed to buy and sell unless and make needless financial transactions. In most circuмstances, unnecessary transactions are to be avoided. If I drove to pick up Chinese take-away on a Sunday or Holy Day, that would be a sin, actually. However, if I invited friends and ate at a restaurant as a shared recreation, it would be permissible. I'm not making this stuff up.
BTW, the catholic site I was referring to was a service that does not sell Catholic items or even "Catholic" service, in the sense that it is a service that guarantees Catholicity. I would be very hard-pressed, to say the least, that this service is at the level of purchasing sacramentals or holy books.
If we needed fuel for our car, that would be acceptable, as well as having to pick up a perscription or something that we might need for our health if there is clearly an immediate need. If while we are out, we decide to pick up a few items for the week, that would be a sin, and depending on their cost and the time spent doing this, it could even be a mortal sin. Just merely forgetting that it's Sunday, and picking up an item or two for yourself out of convenience because you're in their to purchase medicine, would be a venial sin.
Purchasing an item for a gift to be given that day is not a sin, but it is not recommended because it can be a cause for scandal.
Sundays and Holy Days are days in which the faithful are not only to show their devotion to God exteriorly, but also interiorly throughout the entire day. Too many people nowadays make too many excuses to not practice keeping Sundays and Holy Days sacred --- and even going to Mass is not enough to fulfill this obligation.