You basically can't directly send arms. You'd need an export license from the state department, and a lot of items are simply not exportable. I know you're asking about sending $ to buy munitions, but thought I'd add this.

What you can do is to call your congress critter and apply pressure to send UKR more actual munitions. Maybe even suggest that we take all the mothballed A-10s out of storage at Davis-Monthan, make them airworthy, and ship them over for UKR pilots to learn to kill Russian armor with.

They're getting training and arms from Germany, and Biden is sending $600M. I don't think the army needs your money as much as refugees and hospitals do.

You can donate directly to them here

https://savelife.in.ua/donate/

https://savelife.in.ua/en/donate/