As a seller ( and also a buyer) on the LA, I notice that some of the big sellers only update this "paid" info. and mark feedback weeks later . I'll admit, that when the invoice feature first came out, it took me a while to learn how to use it and how to mark the items "paid" and prepare invoices. But now, although I still keep duplicate records on my computer of all sales, I really like the feature of the invoice as computes the math automatically and eliminates mistakes ( hooray!!) and try to send feedback and mark paid when payments come in so I won't get off track. However, I'm just a small seller who sells seasonally.
I do get the frustration from buyers who don't get marked "paid" and who don't get immediate feedback. It has happened to me too, and I don't like it. As a seller, it is frustrating when you try to give fairly quick feedback and then there's a two month delay when you ship. Some of your buyers will post feedback to let you know they got the shipment safely and that they were OK with what they got. ( although like most, I use Delivery confirmation) But some buyers never bother to give feedback either, and although I try to ship plants that I would like to get, I'm left wondering if I did something wrong or if there was something the buyer didn't like since they didn't communicate with me!