Yeah, that threw me off too. This happens in English too though, so I didn’t worry about it too much.
Person 1: “What are you planning to do today?”
Person 2: “I was planning to buy a cake.”
Even though past tense is used, in most cases the second person would mean “I am planning to buy a cake.”
With that said, I’m not 100% sure, so take my explanation with a grain of salt. 
This chapter contains some really great shopping dialog. I like this bit on the bottom of page 71:
全部で４つ欲しいんですけど••• 一つはこのイチゴの•••
(We want 4 all together, but… first, this strawberry one…)
When I was in Japan I shopped in bakeries just like this one several times, but I was never this smooth. I’m totally going to use this stuff the next time I get a chance!
Indeed! And great practice for me for numbers:
p.67 - 4つで十分
p.68 - ケーキ屋って２つあるよな
p.71 - 一つ選んでね
etc!
Here’s a question. Yotsuba-chan (and other kids) often refer to older girls as 姉ちゃん, like at the top of page 72:
これぜんぶねーちゃんが作ったのか？
(Did you make all these?)
Alas, I don’t think I’ll ever have the opportunity to refer to anyone as 姉ちゃん. But what would I use instead? I know better than to use あなた in most cases, but in a situation like this, where I don’t know the sales clerk’s name, how should I address her? 店員さん?
I think for adult women we don’t know we can use おばさん (aunt) or おばあさん (grandmother)… though I think even おばさん might get me in trouble with anybody younger than me (mid-40s). And for men, similarly, おじさん and おじいさん. Is there a similar term for younger men/women whose names you don’t know?
Yeah, calling someone おばさん sounds risky haha. I wonder, if you call someone in their 20s おねえさん will you just sound like a perv?
My best guess is that you should find a creative way to not actually address her. I’m curious what others will say though. Might be an interesting question to put on hinative.
I think it is a great question too. Of course staff usually wear a name tag but I just know it wouldn’t be right to use that! No way. Sadly, my spoken Japanese is even worse than my reading level and I’ve never yet addressed anyone in a shop or anything directly, except with just a すみません. I’m keen to hear the answer so I can go right out and use it! Thank you for an excellent question!
ケーキ屋さん…たぶん？
What’s the opposite of お客さん？
For the most part, it’s just [name of shop]+さん. 本屋さん = bookseller, パン屋さん = baker, ケーキ屋さん = cake baker. Also, Japanese doesn’t use third-person pronouns anywhere near as much as English does. A simple すみません if you need to get their attention will do fine.
Also remember that in the shopkeeper-customer relationship, the customer is the one in the socially superior position. お客様は神様です.
Page 71
Am I misreading it, or does the dialogue here go something like this:
Yotsuba: Which one?! Which one is cake?!
Ena: These are all cakes, you know?!
Yotsuba: What? Wow!
Brilliant!
Yep. Not sure what she thought they were if not cakes. Plastic replicas, maybe?
She’s just really confused about just what constitutes a cake!
She thought a taiyaki was a cake on page 68 and 69!
Page 75
I just love this book so much!
Turning from page 73 (えなー、えなは、どれにした！？) to that picture on page 74 is a delight!
I laugh every time I look at it!
I love how all these jokes and surprises come at the turn of a page!
Anyway, I have a question about page 75.
Yotsuba says: あんましだなドロみたい
Could someone please break this down?
I have no idea about the grammar here! Thank you so much!
I took it to mean:
“It’s not very… it looks like mud”
みたい = 見たい = looks like
Wow! I had no idea! Thank you so much! You’ve just taught me something very useful here! Thank you Rowena!
I got the gist of だな, but didn’t feel I could explain it, so here is Maggie Sensei: http://maggiesensei.com/2015/12/07/how-to-use-the-suffix-〜な-na-なあ-naa/
Basically, “isn’t it”, but it would sound strange to add that in English when she hasn’t completed her thought.
Thank you again! That’s my study for tomorrow! Thank you so much Rowena!
Thanks to you for continuing to ask such detailed questions that prompt further analysis! You’re a great study-buddy! 
I actually picked up / learned あんまり rather than あんまし where I lived (Yamanashi-ken) - does anyone know if this is a dialectal difference? Or is it a neither/neither thing?