Paypal Item Discounted to free, Shipping still to pay - api

Question

I'm setting up Paypal on a site at the moment as an alternative payment method for customers. In general, it's working fine; I can put an order through and it goes through to Paypal's site, shows the correct information, and all is fine. When a user applies a discount to the order, things still work fine, no worries at all.
When a user applies a discount for the full order value (ie. order is for £20, they have a discount code for £20 off) I'm getting the error "Item total is invalid." back from Paypal. The cost being sent across is £0, and the shipping being sent across is £5.
First off, is there anything wrong with the fields I'm sending across, and if so, what's wrong? :) If you need any more info, feel free to ask.
Sample express checkout fields:
Array
(
[METHOD] => SetExpressCheckout
[RETURNURL] => /snip/
[CANCELURL] => /snip/
[NOTIFYURL] =>
[AMT] => 5.00
[DESC] =>
[CURRENCYCODE] => GBP
[MAXAMT] =>
[USERACTION] => commit
[INVNUM] =>
[ADDROVERRIDE] => 0
[SHIPTONAME] =>
[SHIPTOSTREET] =>
[SHIPTOSTREET2] =>
[SHIPTOCITY] =>
[SHIPTOSTATE] =>
[SHIPTOZIP] =>
[SHIPTOCOUNTRYCODE] =>
[LOCALECODE] => GB
[PAGESTYLE] =>
[HDRIMG] =>
[BUTTONSOURCE] => /snip/
[L_SHIPPINGOPTIONISDEFAULT0] => true
[L_SHIPPINGOPTIONNAME0] => Royal Mail Tracked Next Day Ref:/snip (int)/
[L_SHIPPINGOPTIONLABEL0] =>
[L_SHIPPINGOPTIONAMOUNT0] => 5.00
[L_SHIPPINGOPTIONISDEFAULT1] => false
[L_SHIPPINGOPTIONNAME1] => Interlink Express Next Day Delivery (Ord Ref:/snip (int)/
[L_SHIPPINGOPTIONLABEL1] =>
[L_SHIPPINGOPTIONAMOUNT1] => 7.00
[L_SHIPPINGOPTIONISDEFAULT2] => false
[L_SHIPPINGOPTIONNAME2] => Collect In Store Ref:/snip (int)/
[L_SHIPPINGOPTIONLABEL2] =>
[L_SHIPPINGOPTIONAMOUNT2] => 0.00
[L_NAME0] => /snip/
[L_NUMBER0] => /snip (item id)/
[L_DESC0] =>
[L_AMT0] => 8.33
[L_TAXAMT0] => 1.67
[L_QTY0] => 1
[L_NAME1] => /snip/
[L_NUMBER1] => /snip (item id)/
[L_DESC1] =>
[L_AMT1] => 8.33
[L_TAXAMT1] => 1.67
[L_QTY1] => 1
[L_AMT2] => -16.66
[L_TAXAMT2] => -3.34
[L_NAME2] => /snip/
[L_DESC2] => /snip/
[L_QTY2] => 1
[ALLOWNOTE] => 1
[BUYEREMAILOPTINENABLE] => 1
[CALLBACK] => /snip/
[CALLBACKURL] => /snip/
[CALLBACKTIMEOUT] => 6
[REQCONFIRMSHIPPING] => 0
[SHIPPINGAMT] => 5.00
[ITEMAMT] => 0.00
[TAXAMT] => 0.00
)

Answer 1

You can't send an empty or zero amount to PayPal. $0.01 is the lowest you can go.
You can try and work around this by taking a cent from the shipping and adding it to the total or swap the shipping and item charges if the item charge is zero.