Have you really sat down with her and told her that you just want to spend time with her, and that she doesn't need to spend money on you to make you feel good? Figuring out how to have an adult relationship with your mom is not easy if you avoid these types of conversations. You are completely within your rights to tell her you don't enjoy shopping and don't want to shop for things, but because she enjoys it, you are happy to go with to spend TIME with her.
It sounds like your mom is very unimaginative and shopping is a hobby for her, and maybe love=money in her mindset a little bit. If this is something that she can't get over, it might be worth going shopping once or twice during the trip WITH her, but making sure you tell her that this is shopping for things for her, not you, because you don't need anything and make sure to remind her if she tries to get you to buy things. I mean, it's fine to go and try on things for fun, or window shop - I do it all the time and it can be fun if you do a few hours and then go get a nice lunch after. You don't have to spend money to enjoy looking at pretty things, or trying on a few pairs of shoes or a dress or two, and doing lots of people watching. If she gets angry about this, that's on her, and I would gently, and as kindly as possible explain that you just enjoy spending time with her and really don't want or need any of the things you're looking at - remind her as often as need that you're having a nice time but no, you don't need "X" or whatever. I would not let her goad you into doing things you don't want to do (spend your money or her money on things you don't want/need). And remind yourself - you are allowed to say no - even to your mother - because you are also an adult. It doesn't have to become a fight, or cause hurt feelings, just stay upbeat and pleasant.
I would NOT suggest buying things and then returning them. It is dishonest and wastes your time and money and doesn't solve the problem of you being an adult who does not like to shop and being forced into it so you don't have to have a difficult conversation with your mother about how you're a grown ass woman who is allowed to do things she wants. Don't be that person. Especially if you ever want a truly honest, open relationship with your mother.
And when you say you should just eat the expense... do you mean you are buying HER things? If that's the case, then yes, I would tell her that money is tight and you can't afford any shopping sprees.
If the past trips involve you buying things for yourself and she buys things for herself, then if it was me, I'd likely not buy anything and not criticize her purchases, because she's a grown woman and you already said this is not a normal outing for her. Go with her, enjoy her company and let her shop. She can't make you buy anything for yourself and just keep in mind the idea of: "I don't need anything and I'm not seeing anything I really want either, but I am glad to help you pick out a few things and I love spending time with you."
But I would also make sure that the shopping trip is one outing to the big city with a lunch and maybe a manicure - sort of a girl's day out thing, and then fill the rest of the trip with pleasant activities you can do at home, a few outings to fun things around your area, and just chatting, playing games and whatever group activities you enjoy and want to share with your parents. So the shopping is one very minor blip on this trip and confined to one morning or afternoon.