Could you please select 〇〇 yen worth of these for me?
I would like to buy (about) 〇〇 yen worth of these. Could you please decide for me?
I'd like to buy 20 dollars' worth of flowers. Could you please choose for me?
お花を買う時等よくありますよね。
「私のために選んでくれませんか?」 => Could you please select (...) for me?
Could you please choose (...) for me?
Could you please decide for me?
○円分 = ○ yen worth (of)
○ドル = ○ dollars' worth (of)
"Will you choose some items under ○○Yen for me?"
「私のために○○円以内で商品を選んでくれますか。」
何のために見繕うか、という部分の説明をしたい場合は、例えば
Will you recommend some items for
my wife’s birthday? 妻の誕生日用に…
a thank you gift? お礼のギフト用に…
a wedding gift? 結婚のギフト用に…
いくつか商品をおすすめしてくれませんか。
と尋ねてもいいと思います。