It depends on how you know this is going on. If she's returning them and demanding a gift card or cash reimbursement, are you getting a notice from the store because it was on your account? Or is she announcing to you that she has done this and is she acting irritated that you are "creating work" for her by making her go to the P.O.? Or is a third person (like your son) telling you about it?
If she's saying it directly to you, then I'd calmly say, "You can rest assured that this won't happen again." And stop sending her things from stores.
If you get some notice on your credit card about a return, you can either ask her about it and ask what she would prefer, or just stop sending gifts you buy on line. Maybe you don't know her taste, maybe she's one of those who gets offended if people shop from home vs. physically going out to browse. Or, if it's all based on a prior argument, you can just skip the discussion about it, and take the not-very-subtle hint that these gifts are not appreciated. If you keep buying, no one is benefiting - she's angry at you or disappointed at the gift choice, and you're getting your feelings hurt. There's no reason to keep doing it the same way if it's not working out!
If someone else is informing you of this, it's either someone who wants to get you to stop, or who is trying to hurt your feelings and drive a bigger wedge between you and your daughter-in-law. But still, assuming it's true that she is returning the gifts, what's the point in continuing?
Trying to "carry on as normal" makes no sense if you have wildly different taste than your daughter-in-law. Switch gears. Do you know enough about them as a couple to know what they like to do? Is there a restaurant they love? A movie theater they frequent? Send gift cards or movie passes for those locations - it shows you know what they like but lets them choose the day and the selection. Do they enjoy a museum in their area? Buy them a year's membership. Do they garden? Get a gift card from a particular, local garden center. Or send cash. Or send something "perishable" like a plant, flowers or perhaps a food assortment that cannot be returned. But only do that if you are sure they will use it and not resent you further.
If this rift between you and her is that big, I'd work on repairing it or at least apologizing (whether or not you think you are wrong) rather than trying to ignore it by buying gifts she's going to return just to annoy you or show you her displeasure in general. And finally, if you are not appreciated and not thanked for what you send, just cut way back and just send cards.