The Wonderful Garden by Edith Nesbit

‘We simply must write to Aunt Emmeline,' said Caroline earnestly. ‘I've got three new pens and some scented violet ink. I got it at the shop yesterday; it's lovely. And I've been counting up the picture post-cards she and Uncle Percival have sent us. There are forty-two, and twenty-eight of those have come since we wrote last.'

‘I'd almost rather not have the post-cards; they make you feel so horrid when you don't write,' said Charles. ‘Suppose _we_ send picture post-cards. You don't have to write nearly so much.'