X就是了: just X, that's all there is to it
【四72】口语格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
X就是了 closes a sentence to say “just do X and that's that”: X is the easy fix, and there is nothing to worry about. 缺什么，到那边再买就是了: “if you're missing something, just buy it there”.
The tone is spoken and reassuring, softer than an order. You play a problem down (你直接说就是了: “just say it, don't be shy”) or give in gracefully (好，我去就是了: “fine, fine, I'll go”). So X is a suggestion, a plan or a promise, never a report of something that already happened.
Don't mix it up with 而已 or 只不过, which belittle a fact (“it's only…”), nor with 就是 (“precisely”). Think of English “all you have to do is…” or “just…”. It sits at the very end, 了 included, and can follow a condition: 不想去，不去就是了.
Examples
缺什么，到那边再买就是了。
Quē shénme, dào nàbiān zài mǎi jiùshì le.If you're missing anything, just buy it over there.
你直接说就是了，不用不好意思。
Nǐ zhíjiē shuō jiùshì le, búyòng bù hǎoyìsi.Just say it straight out, there's no need to feel awkward.
你别生气，以后不跟他说话就是了。
Nǐ bié shēngqì, yǐhòu bù gēn tā shuōhuà jiùshì le.Don't be angry: from now on, just don't talk to him.
好好好，我去就是了，你别再说了。
Hǎo hǎo hǎo, wǒ qù jiùshì le, nǐ bié zài shuō le.Fine, fine, I'll go: stop going on about it.
如果你不想去，不去就是了，没人会怪你。
Rúguǒ nǐ bù xiǎng qù, bú qù jiùshì le, méi rén huì guài nǐ.If you don't want to go, then don't: nobody will blame you.
Common mistakes
✗ 你说就了。 → ✓ 你说就是了。 (the formula is fixed: 就是了, in that order).
✗ 他已经走就是了。 → ✓ 你去找他就是了。 (X is a suggestion or a plan, never the report of a fact).