Not super good taste to make fun of disorders like this.

There's a lot of lines to Umeda lol

Generally speaking, text online is encoded in a giant huge enormous standard called Unicode. Unicode is cool because it definitionally encodes every possible character in every language in one standard. Uni = single, code = code. However, Unicode is not quite so cool because to do this, it needs to

1. Be huge

2. Get updates from time to time to add new characters people might want to encode

The person I replied to probably copy+pasted the best-looking arrows they saw off some webpage online. This would have been fine, except that the arrows they happened to choose were relatively new to Unicode (from 2014), with relatively little support in fonts. That, by the way, is the other, other issue with Unicode — just because it theoretically supports a character, doesn't mean the specific font you're using on your computer/phone/smart fridge does too. Your font could have been created before the character was added to Unicode, or you could be using a version of the font released before that character was added, or the font creator might just not care about that random arrow you want to stare at with your eyes. So, to support more devices, I chose old symbols for the arrows (technically not even arrows in my case, they're just triangles — from all the way back in 1993!) and old emoji for the A and B (because most devices these days support emoji and I thought they looked neat here). Fun bonus fact, those A and B symbols aren't even designed to represent buttons — they're for blood type, which Japanese people (Japan being the origin of emoji) have historically been obsessed with. It's kind of like the Chinese zodiac, or star signs. But I digress.

This is probably more info than you wanted, but sometimes answers just be like that.

Alternate symbols because the directions showed up as rectangles on my phone:

I mean there's lots of options for that. You could order one book ahead in the series as you're reading, you could pick an arbitrary point in each book (like halfway through) to order the next one, you could buy the books in a brick-and-mortar store, or you could just exercise patience.

How often is 2-day shipping really a necessity?