くれりゃ is a colloquial, contracted pronunciation of くれれば.
For this kind of contraction, see this thread:
What is the meaning of りゃ in this phrase?
注文してくれれば breaks down to...
注文して -- "order", the て-form of the する-verb 「注文する」
くれれば* -- "if you do ~~ (for me)", the conditional form of the subsidiary verb 「くれる」
(*ば is a conjunctive particle)
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This is one of the Seven Wonders of English Education in Japan of which my poor English is a product.
Long story short, I can guarantee as an average native Japanese-speaker that 「～～ほうがいい」 is closer in both meaning and nuance to "should" than to "had better".
I was taught the complete opposite in junior high school in Japan (I was indeed taught "had ...