Cash Register, displays menu and allows products to be rung in. Currently requires input validation

Cash Register

def menu(hash)
  i = 1
	hash.each do |k, v|
		puts "#{i}) Add item - $#{decimal_output(v[0])} - #{v[1]}"
		i += 1
	end
	puts "#{i}) Complete Sale" , "" 
end

def prompt(string)
	puts string
	gets.chomp.to_i
end

def line_totals(hash, quantity, selection)
		hash[selection][2] += quantity
end

def subtotal_display(items)
	items.length.times do |i|
		puts "$#{decimal_output(items[i+1][2] * items[i + 1][0])} - #{items[i + 1][2]} #{items[i + 1][1]}"
	end
end

def decimal_output(num)
	return (sprintf('%.2f', num))
end

items = { 
	 1 => [5.00, "Light Bag", 0],
	 2 => [7.50, "Medium Bag", 0],
	 3 => [9.75, "Bold Bag", 0]
 }

selection = 0
subtotal = 0
tender = 0

menu(items)

loop do
	selection = prompt("Make a selection:")
	break if selection == "#{items.length + 1}".to_i
	quantity = prompt("How many bags:")
	subtotal += (quantity * items[selection][0])
	puts "Subtotal: #{decimal_output(subtotal)}"
	line_totals(items, quantity, selection)
end

puts "===Sale Complete===" 
subtotal_display(items)
puts "Total: $#{decimal_output(subtotal)}"

until tender >=  subtotal
	payment = prompt("What is the amount tendered?")
	tender += payment
	if tender < subtotal
		puts "You owe me $#{decimal_output(subtotal - tender)} more!"
	else
		change_due = tender - subtotal
	end
end

puts "===Thank You!==="
puts "The total change due is $#{decimal_output(change_due)}"

puts Time.now.strftime("%m/%d/%Y %I:%M%p" )

#User Stories: 

#As a Cashier
#I want to see the menu
#So that I can input the correct item price or complete the sale

#As a Cashier
#I want to input quantity of items purchased
#So that I can calculate the total

#As a Cashier
#I want to see a bill
#So that I charge the customer

#As a Cashier
#I want to know how much change to give
#So I can finish transaction with a zero balance

#Acceptance Criteria

# Input must be valid
# Input options will be seen by cashier
# Item and quantity will be input by cashier
# Final item and quantity subtotals must presented when requested by Cashier
# Change due will be calculated for Cashier
# Change due may not be negative