OK, I think I understand this a little better now. I don’t think I realized that your sheet was built from a unique formula. I think the rows are being created by google when the UNIQUE formula hits the end of the sheet and automatically adds the 500 rows as a buffer. I think what we have here is a non-issue. Since your are only building the sheet from a unique formula instead of a from a form button (this is what I thought was happening), you really don’t have to worry about the extra rows unlike using arrayformulas in a sheet that filled via the Add button or a Form button. In that case, the new rows would be added to the very bottom of the sheet as a new row. The Unique formula that you are using will always keep the data at the top of the sheet. The only thing I could not duplicate is the number of used rows in Glide jumping to 500/500. For me adding a new item to the cart only added 2 rows. I’m guessing you maybe had a formula in your sheet that I couldn’t see in your GIF that wasn’t writing a blank value if that row wasn’t filled from the unique formula. I’m assuming you maybe fixed that. Even with an arrayformula and the extra rows, if you are filling them with blanks when the formula doesn’t apply, then Glide will not recognize them as filled rows.
I think the 500 additional rows in your Order Summary sheet should not be a problem and should not affect anything in your app. It’s just the google sheet expanding for any potential future values.